In the latest episode of the Blizzard podcast, Diablo III senior artist Mike Nicholson discusses organising the inventory system by size and giving skill buttons a World of Warcraft makeover.
The Diablo series has always had inventory issues. I'd say 90% of "Town Portal" scrolls are used to head back to town and offload your inventory with a merchant. Diablo III was initially going to feature an inventory where all items were icons of the same size, but now they've split them off, with a bag for small items, a bag for large items, and a bag for quest items. It really isn't as confusing as it sounds.
Current design right now is to have bags. You'll get bags and they will expand, you know kind of like in WoW, except you're not going to open up separate windows. You'll start off your inventory with say... you know, eight slots, right, and then you'll get a new bag and that has ten, so two more slots will open up within that tab, but you'll never have multiple tabs. Like you won't have two or three "large" tabs.
Nicholson takes a great deal of inspiration from Blizzard's massively multiplayer online game, citing it as the reason behind Diablo III's more colorful skill system as well.
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 while it will definitely piss off those Diablo fans that have memorized power symbols using stacks of flash cards, the coloured skills were necessary to keep track of all the new and different skills being packed into Diablo III.