Weapon switch hotkey
The weapon switch hotkey was added to Diablo 2 in the expansion pack, and was a highly popular and much-used/abused feature of the game. This hotkey, a standard feature in virtually every RPG at this point (many games allow full outfit swaps, and/or cycling through multiple weapon sets), allows characters to exchange their currently equipped item (usually just their weapons) for another weapon or weapons with the single click of a button.
There are no current plans to include a weapon switch hotkey in Diablo 3, a fact that most experienced Diablo 2 players greeted with shocked horror.
The planned exclusion of this feature was most recently confirmed by Jay Wilson in an October 2009 interview. 
- Diii.net: Weapon switch hotkey?
- Jay Wilson: No. We took that out.
- Diii.net: Oh, I loved that feature in D2.
- Jay Wilson: Mostly we took it out because the only examples we could come up with of how people used it were somewhat exploitive. Most frequently people used it by accident and wondered where the freak my weapons went. So it didn’t seem a useful feature for anyone but a very small portion of the audience that used it to swap items for magic find purposes, which seemed not a great super fun reason to swap weapons. There were probably some other uses, which I’m sure I’ll be reminded of in the forums.
- Diii.net: There were a lot of specialized builds that found it essential to use two sets of weapons. Hybrid Amazons, chiefly. Used a bow and also a javelin/shield.
- Jay Wilson: Yeah.
Jay's bland dismissal prompted that interviewer to launch into a (written) rant about the idiocy of removing that feature when the interview was posted, and the news thread had soon attracted more than 75 comments, most by fans weighing in on the issue.
- Will there be a quick weapon switch (‘W’ in D2) implemented in Diablo 3?
- Bashiok: Just like Diablo II we’re saving secondary weapon/shield slot and hot key for the expansion.
- Kidding! ... probably. We don’t have it now, but that doesn’t mean there won’t be one. I don’t think it’s an irrelevant question, maybe just not the most pressing one.
- I don’t think it NEEDS to be there. It’s not really an iconic part of the game. The gameplay dictates its necessity, and if there’s no reason to hot swap weapons mid-fight, then having it there just clutters the UI.
- Personally I just used it in Diablo II for extra item storage.
It's apparent from these and other past comments that neither Jay or Bashiok (or anyone else on the D3 Team?) played a great deal of advanced Diablo 2, and expert D2 players have been known to despair at the inexperienced comments coming from the team making the sequel -- an unfortunate side effect of the almost complete team churn caused by delays, disagreements, and the closure of Blizzard North and ultimate restart of the project at Blizzard Irvine in 2005.
Pros and Cons
The weapon switch hotkey is just a convenience; after all, players can simply carry extra swords, shields, suits of armor, etc, in their inventory, and change into them as the situation demands. The whole point in having a special hotkey for this is to allow players to do it very quickly, as is required in a fast-paced action game like Diablo 2 or Diablo 3. Since the feature is optional to use, and simply adds more control options, it's odd that the D3 team would want to remove this one, since they're adding so many other features in Diablo 3 to make the game easier to play.
The D3 team's stated opinion that it's an exploit came as a shock to experienced Diablo 2 players, most of whom had used it constantly in that game, and considered it an integral aspect of the play experience.
With some uses of the switch, the accusation of "exploit" is hard to argue against. None of these are "exploits" in terms of being cheats; they're just smart uses of a game feature. The most common trick with the weapon switch hotkey was to change to a Magic Find boosting weapon/shield an instant before killing a boss monster. This was a very useful technique, a fun technique to master, and added to the item game, putting a much higher value on +MF gear for all characters. That said, it was physically unrealistic, as well as favoring of some characters over others. A spell-caster could easily finish off a monster with any type of equipment in his/her hands, while combat characters could not finish off an Act Boss with a wand and shield at the ready.
Other arguably exploitative uses of the weapon switch were to cast spells enabled by unique/runeword weapons, or to boost your character's skill levels before casting a buff.
Many players simply used the switch for extra inventory space, which was a convenience, but not an exploit.
Other Diablo 2 characters or play styles quite required the weapon switch to function at all, or at least without the frustration of constantly opening the inventory window and clicking and dragging weapons around. Including that feature enabled numerous builds that would otherwise have been almost unheard of.
The extremely popular hybrid Amazons are the most obvious examples, with a bow on one switch, and a javelin/shield on the other. Other characters in various patches, notably Paladins and Fury Druids, required a regular melee weapon, and a second one with high elemental damage on their switch, to deal with Physical Immune monsters.
Furthermore, many low level characters, especially those without quality twinked items, were wise to use the weapon switch to change between different weapons in the early going. A blunt weapon for fighting the undead, a sword for animals, a throwing weapon for fleeing enemies, etc. This sort of use will be just as necessary in Diablo 3, and numerous expert Diablo 2 players found themselves already missing this option even during their brief Diablo 3 playtime at Blizzcon in 2008 and 2009.
In or Out?
At this point, there's no telling if the feature will remain out, or return. The game has a long way to go in development.
One mitigating factor comes from the different types of mods on Diablo 3 items. The D3 Team has said they do not want to include +skills items, and if there are no items that add +3 to warcries, or enable specific skills, then a great many of the exploitative uses of the weapon switch hotkey vanish right there. There is also a design goal to require spell casters to use weapons that boost their killing power, which removes another easy +MF exploit from Diablo 2, since mage characters could kill easily no matter what their weapon's stats.
One compromise solution could be to include a weapon switch hotkey, but putting some sort of cool down timer on it. Switching could only be done so often, or would require a few seconds to take effect. After all, a character would logically need a few seconds to sheath a sword and pull out a mace, or a halberd, from his invisible, all-encompassing backpack.
This is the sort of feature that gets evaluated in more detail in the later stages of game design, when the controls are fine tuned, and beta testers get to put in their input, so it's quite likely that we've not yet heard the last of this issue.