From Diablo Wiki
Icons represent skills and other game controls. They must be visually attractive while giving information about the skill, spell, or other function they represent. A surprising amount of design work goes into icons, while the team reworks them and figures how best to fit them into the interface's main hotbar, and the various menu windows.
The icon design process was discussed during Blizzcast #8, in March 2009. 
- Bashiok: Icons are an iterative process. A lot of fans at BlizzCon saw the skill icons. And there’s been some talk in the community that the skill icons are definitely a departure from what we saw in Diablo II. Which were these gold runes, almost, I guess. So what was the decision there to make a change to the skill icons, and do, y’know colorful skill icons?
- Mike: I am unabashedly a fan of WoW, I’ll probably catch flak “oh you’re making it look like WoW oh no!” but I love them, I love the WoW icons. And y’know what we’re trying to do is see, ok what works about the WoW icons? They’re illustrative; is there a way to try to maybe bridge the gap. You know what worked about the more symbolic DII icons, and what works with the WoW icons, and trying to find a way to put the two together. And that’s really what I’m trying to do is to try to make them more symbolic, so they’re not necessarily little murals, but they are painterly. So they are colorful because you’re going to have a lot more of them than you would in DII. So you have the hot bar at the bottom, and you want that to look colorful enough that it’s interesting, but not to the point where it’s an eyesore. And quite honestly I’ve made many iterations on the icons, and there’s still many to come. It ends up being a balancing act, you know I test them out what they look like next to each other, and if they’re too colorful – tone it down.
- Bashiok: One of the things I like about them is, without even looking at your hot bar you can kind of see, oh this one looks blue and cold it’s probably an ice spell of some type. So without even mousing over it to see what it is I can tell it’s at least of a specific magic or damage type or skill use...
- Mike: You know one of the best experiences I’ve had at Blizzard is the iterative process, which some artists don’t like, but I personally love it. Because it lets me test stuff out. For a while I was trying out, ok let’s do color theory… ok so all the berserker skills will be this tone, and all the battle master will be this tone. And you know on paper it looked good, sounded good, ok it’s a nice theory but what we found is when you put them on the hot bar, if you went down a certain tree now everything’s the same color at the bottom and it’s hard to tell them apart. You know, ok I get a gold star for being clever, but then I lose points because it’s not playable. So you know you go back once again, scrap it, and reevaluate it. But I love that process.