D2 rewards a player for playing the game
When Diablo 2 came out, it blew Diablo 1 out of the water. It did everything better than Diablo 1 and added a ton more content. Diablo 3, on the other hand, somehow has less content than Diablo 2 and, frankly, doesn't do things that much better. Unless of course graphics are really, really important to you. To me, it feels like Diablo 2 was a huge step forward while Diablo 3 is a step way over to the side and a little backwards. There's so much stuff in Diablo 2 that just isn't present in Diablo 3.
D2 rewards a player for playing the game. 100% of the legs you farm are useful, at least, for a beginner.
D3 does little reward, 99% of the legs you farm are crap (even for a beginner like me). Therefore in order to gear up, in a reasonable manner, you need to either use RMAH or farm AH. Diablo 3 Gold.
No real PvP, terrible itemization, no breakpoints, no skill points, no stat points, no max block, no weapon switch, no runewords, bad mercs, horrible social interface, linear gameplay, abysmal story, power ranger villain killed cain, secret level is an insult to the players for saying the graphics are too colorful, Blizzard doesn't care what Diablo fans want, skills suck, small amount of builds, no ladder, no offline mode, ability to bind 6 skill hotkeys instead of 17, cooldowns cooldowns cooldowns. There's more, but it would literally be easier to say what D3 does well. Graphics and game engine. That's it. Literally.