From Diablo Wiki
A mod, mods, or the process of modding refers to player-made game modifications. Modding has always been popular amongst the Diablo community, despite Blizzard's never including any helpful map-making or other utilities. The D3 Team will not be officially supporting modding in Diablo 3 either, though fans will surely find workarounds, as they have with the other games in the series.
Mods are unofficial changes that often make the game incompatible with the official version. Modding ranges from from minor changes like changing the stats on weapons or monsters, up to "full/total conversions" where the entire game is reworked in very substantial ways.
Blizzard has not done much to support modding in the Diablo community, since the company makes the games with proprietary tools that are not user friendly, or in the public domain. Despite these obstacles, players have created some amazing mods, greatly extending the play experience of Diablo and Diablo 2.
Sadly, the would-be modders will be running uphill again with Diablo 3, since the D3 Team has no plans to make the game mod friendly or to release any tools that assist the modding community.
Official Comments on Modding
- "We don't have a lot of plans to do that. It would make our lives so much harder" he said, adding that despite the fact that he and many other designers got their start in modding, the team takes a strong stance on the hacking of their games.
- About mods in Diablo 3, they've been really crucial to long-term playability. What are you planning to do about them?
- Jay Wilson: We considered the idea of doing mods, but decided a map editor was too ambitious. The randomness does not lend itself to create user mod tools. We value mods, but "for us it was really a choice of randomness or in-user mods."
- Diablo has never officially supported modding although they haven't really banned it. Is there any thought towards modding going into this or even map editing? Have you guys thought about including a map editor?
- Leonard Boyarsky: We talked about it early on and we considered it but the way we put together our maps and the fact that it's random ... it's very artistic-centric. And, on top of that, the fact that it's so random it's like, would people just change the random number generator? You know what I mean? [laughs] We don't hand-build our dungeons anyways, but the way we build our maps kind of makes that prohibitive. But we're always looking at what the end users might want so we did look at including a map editor and we just said that it's never been a big part of Diablo. So we didn't feel it was necessary.
- Kevin Martens: We're certainly not opposed to modding it.
- Leonard Boyarsky: Yeah, we're not going to put in things that, "Oh my God, you can never mod this!" If somebody comes along and makes this great editor and makes this great mod...
- Kevin Martens:At this point, we're knee-deep in just making the game. That's sort of at the periphery of our discussions at this point.
Blizzcon 2009: 
- Slashdot: Speaking of the artistic side of the house, are you looking to leverage your community for artistic injections into the Diablo III universe, like custom levels, modding, or even total conversion mods that just utilize the Diablo III engine?
- Leonard Boyarsky: We discussed that early on because Starcraft II is doing so much modding support. But when you look at the style of game that Diablo is, it is based around a lot of random content. So when you look at it from that standpoint, someone might be able to make some very specific content, but the basis of what we're providing for the player is a random system. So are they just going to provide a different random system? Also, the creation of our art is very intense in terms of not only the talent and technical expertise required to get it into the engine, but manipulating it and using it with our tools. It would take a lot of work to make that friendly for the end user who didn't have a programmer there to help them figure out some of the finer points. We just didn't see the bang for the buck in doing something like that and it was never really a big part of the Diablo fan base. Having said that, if someone comes along and takes the Diablo engine and makes a fantastic game out of it, more power to them. We just didn't feel that was where we could add the most value for the players, because that just isn't what the community is about.
Fan opinions are mixed on the fact that Diablo 3 is not going to support modding. (There will certainly be modding, since there was a great deal for Diablo 1 and 2, despite those games not "supporting" it in any official capacity.) Many fans look at this as a money grubbing control issue by Blizzard, while others aren't interested in mods and are fine with the team not spending/wasting any time developing for them.
Diablo 2 Mods
Brother Laz's D3 Team.has grown justly famous and popular. This mod makes massive changes to the game, reworking every skill and introducing a great deal of new artwork and special effects. It's been commented on favorably by the
See thefor more information on this subject.
Modding Tips from Laz
Brother Laz is a major personality within the Diablo II modding scene, and he has a few tips for people wanting to make mods:
1/ Start small. Don't attempt a total conversion as your first mod. You'll just give up when you're making no progress.
2/ Never release your first mod. You'll end up getting no downloads and quitting the mod business. I did release my first mod, and counted my lucky stars I released it under a different nickname 'just in case'. Look for D2 Inferno by Myokai to see why.
3/ Use the file guides, the tutorials and the forum search at d2mods.com. You can learn so many things without having to post anything at all.
4/ Make sure you actually have the time to complete a mod. If your mod ends up cancelled due to lack of time, all the time you already invested in it is lost.