Diablo 3 was too simple and kinda deserted the hardcore community that was built around Diablo 2. Needless to say, this was probably done to attract new customers rather than keeping the hardcore fanbase at bay.

Character builds say it all mostly. In Diablo 3, you can unlock every skill, and every rune, meaning every barb is the same as every other barb, and so on. In Diablo 2 though, each character had 3 different skill trees, with 10 skills. Each skill could be leveled up to Level 20, although there were only a total of 108 skill points you could get (1 skill point per level, 99 being the max level), so basically you had to choose how to build your characters. this made for some insane builds, some insane PvPers, and gave the game a lot more replay value. Not to mention the combat was much better. Oh, and rune words, weapon/armor sets, and the unique items were all pretty fun too. It's a game you gotta try, though you need to get the expansion.