From Diablo Wiki
Respec is slang for respecialisation and refers to a game feature that allows players to partially or completely reallocate the points they have spent in various skills, 'talents', 'attributes', 'feats' or other character values. Most modern RPGs (including World of Warcraft) have respecs of some kind.
Respecs did not exist in Diablo 1 or Diablo 2. Diablo 2 eventually added limited respecs, but not until 2009 when the original developers were long gone and the Diablo 3 devs authored respecs in Diablo 2 partially as a test bed for their planned Diablo 3 respec option.
Diablo 3 and Reaper of Souls allows for almost complete freedom in respec'ing. There is no limit to the number of times skills and passives (and Paragon Points) can be respec'ed, though when this can be done is slightly limited. Respecs done in town take effect instantly. Respecs performed while in the dungeons have a short cooldown (15 seconds) before the new skill becomes active. Skills and passives can not be respec'ed while in combat, or while a character's pets or follower are giving or taking damage.
There was controversy about the concept of Respecs in Diablo 3. Some fans felt that making respecs too free (freespecs) would hurt replayability and longevity, with no incentive for players to ever reroll. (Hardcore excepted.) That debate continued for some time after release, but eventually respecs became accepted and by 2014 and Reaper of Souls the issue had vanished from the community.
Diablo III Respec Timeline
Respecs were always promised as a Diablo 3 feature. How readily they could be enabled varied during development, with a few periods of complete or nearly complete limits on respecs. When the Diablo 3 Beta test began respecs could only be done in town, or after clicking a special shrine, but upon release respecs were more readily available.
A timeline for Respecs during development of Diablo 3:
- Diii.net: Respecs. Cost, gold mechanism, any specifics?
- Jay Wilson: We will have respecs. We haven’t determined the mechanism yet, but we’re definitely trying to do something different than a wholesale pay a token price and get all your skills back. We want to have a higher price than that.
- We’re still working on the respec system for Diablo III. For those who haven’t seen the Diablo II 1.13 patch the respec system in there, which is still on the PTRs by the way (or maybe not), is you can get three different respecs by completing the Den of Evil quests and then any additional ones beyond that by picking up item drops from bosses and using the horadric cube to combine them.
- For Diablo III we don’t have it really nailed down yet but some of the main points we do want to hit are that it’s more than just a gold cost for changing your mind. Although that will be a major component of it. That it’s targeted for specific skills so you’re not resetting your entire skill tree, but you’re able to target specific skills to reset those ones specifically. To also scale the cost of respecing so early on in the game it’s cheaper and easier to respec and as you go farther into the game it will be harder and more expensive.
- And all of those things together make it a very challenging system to implement, hitting all of those notes. And those are all pie in the sky, I guess, hopefuls for the respec system we may not get all of them in. We don’t have any specifics on what the actual respec system will have but those are sort of what we’re shooting for and of course it’s all up in the air still.
Bashiok again spoke on respecs in September 2010.
- Bashiok: We don't plan to have full resets, they'll be per-skill (if it goes as currently planned), and we don't plan to allow them to be easily obtained. Probably similar to the Diablo II 1.13 respecs a couple will be easy, or granted through quest rewards, but the idea is to involve some amount of rarity, cost, something. What that balance will shake out to mean... I don't know.
Respecs and Replayability Debate
One of the main arguments against respecs, or at least against free/easy respecs, is that they will hurt replayability. As the argument goes, instead of players making new characters and trying out whole different play styles, they'll just reset their character and switch around their skills, whenever a new build becomes popular, or a patch changes existing skills.
Against that the pro-respecs people argue that many players don't have time or interest to reroll a character from scratch. Thus they stop playing, or keep playing their gimped character and don't like it. Whereas if they could just change that character's skills around, they'd be much happier.
Bashiok delved deeply into this debate with a long series of forum posts in September 2010. His arguments are not necessarily those of the rest of the Diablo II developers, but he's clearly been a witness to many internal team discussions about this issue, so we can take his logic as somewhat representative of that of the D3 Team. 
- Amount of skills in Diablo III, with all player runestone variants included, is currently topping 700. That's about 140 rune combos per class! neat!
Bashiok: It's staggering really, even to me still. Those are all unique in their visuals too, which is important to note. Sure some of them may be simple color shifts but most of them are unique mechanics with completely unique visual effects. It's crazy. Big up to our effects guys.
- I know, I was thinking about that too.. and this is why I cant even fathom how some people think 10 characters will be enough!!! Did they even PLAY diablo2
Bashiok: Explain please. With respecs and runes being swappable, how does that necessitate a ton of character slots?
- Your own words say that respecing will be very limited.. if I want to make a new character with a different spec Ill eventually need to be deleting characters (Pretty quickly may I add) at a 10 limit.
Bashiok: Actually, on that note, who ever said there was a 10 character limit? Not to say that's wrong but, I'm guessing Jay threw out a number somewheres? Also you're quoting me from how long ago? Those seem like very old quotes. Have you played Diablo II 1.13? We added a respec system. See how that works and you'll get an idea of how we think we'd like it to work for Diablo III.
- Yes, I did play that and it contradicts completely everything stated thus far for Diablo3 respecs.
Bashiok: Nuh uh!
- Complete character wipe
Bashiok: Ok you got me there, but that was just for implementations-sake. The way the trees work in Diablo II you can't have pointed resets. But we can with how the skill system is set up in Diablo III.
- Very easy to obtain, not rare or valuable at all.
Bashiok: Well that's just balance.
- I also do not like that because it essentially cuts the replayability of Diablo2 down sharply.
Bashiok: It cuts down replayability in the sense that you don't need to roll a new character when you want to try a different build. And yes, it absolutely does that, and ... that's the point of having respecs... I mean part of it is giving people an out if they screwed up and bought the wrong skill or something, but the main reason is so you can play around with your character and not have to start a completely new one just to try a variant out.
- If respecs happened when Diablo2 came out I doubt anybody would be playing anymore.
Bashiok: Potentially. But I would argue that it's about the items, the randomness of the drops, and the character builds are simply a means to roll the dice more effectively, and use them in more effective ways.
- You don't need to make a new character and level it, find new gear for another character or do any of these things.. if youre now saying Diablo3 respecing could be like this.. well.. im pretty worried.
Bashiok: Why are you worried?
- Well you say it should be rare, then you say how they were put in Diablo2.. Which was my whole point about that. A contradiction of balance
Bashiok: Oh I was referring to the method of obtaining them. It's one we like. But yeah people steam rolling bosses in Diablo II means the drop rate was probably set too high.
- Yes, it cuts down on a major part of gameplay which is why I do not like full easily obtained respecs
Bashiok: We don't plan to have full resets, they'll be per-skill (if it goes as currently planned), and we don't plan to allow them to be easily obtained. Probably similar to the Diablo II 1.13 respecs a couple will be easy, or granted through quest rewards, but the idea is to involve some amount of rarity, cost, something. What that balance will shake out to mean... I don't know.
- And when you only need one set of gear (Since most characters use the same gear regardless of build on Diablo2) for a character you can respec at a whim you will min/max much quicker and will need too roll said dice of item finding a lot less.
Bashiok: That's a lot of assumptions about how things will work. Let's wait a while and talk about it when these assumptions have defined rebuttals. I could counter your assumptions with our hopes and dreams but that doesn't mean much, IMO. [/BLUE]
Since that time the team has revealed that characters will only get up to seven skills at a time in Diablo III. And since none of them have pre-requisites, stripping all the points out of a few of them and investing those into other skills seems like a fairly easy change. That's not all that will be required to remake a character, though. And it's the other aspects of respecing a character that make it clear that most players will have more than one character of a given class.
One or two skills respecing might not greatly change a character, and it will be fun just to throw max points into a new skill just to see what it looks like. But to do a full on character remake, switching out of your main skills to different skills that support an entirely different play style, will be a big deal.
What steps would be required to remake a high level character, one specialized for a particular type of play style?
- Respec skill(s).
- Obtain new runestones to grant the desired bonus effects to your new skills.
- Respec supporting traits.
- Change weapons and armor to match the new play style.
- Change socketed gems to match new play style.
- Change the charms in your talisman to match new play style.
When you consider all that, it might be easier to just make a second character in the desired style, so you could play both of them right after each other, perhaps with some equipment overlap, easily enabled via the shared stash.
Diablo II Respecs
In Diablo II, prior to v1.13, characters could not alter any of their attribute or skill points after they were initially placed. Not even a single point, just to fix a mis-click.
This change to D2 was largely instigated by the Diablo 3 developers, and Jay Wilson stated during an interview in August 2010. 
- in.Diablo.d3: If you could only change one thing from D2 to make D3. What would be the thing?
- Jay Wilson: *happy and eager* I already did it. Respecs. My design team worked with the D2 patch guys. We highly encouraged it, they did the work.
Diablo I Respecs
In Diablo I spell levels were gained by reading books, and higher level spells were always better (aside from a bug with Mana Shield in earlier versions, and potential sprite limits with Chain Lighting). There was therefore no need for respecs, and they weren't anything any player ever wanted or thought about.