Diablo 2 used randomly generated dungeons the same as D3. It might be your opinion that the artwork itself is better looking, but saying that D2 was handcrafted and D3 isn't is factually wrong. There are parts of both games that used handcrafted dungeons and everything is randomized. That's part of the appeal of Diablo. It's never quite the same experience twice.It's my subjective opinion. Diablo 2 was all hand-drawn, intimate, mood setting art. Look at the buildings and landscape in act 2 for instance. It seems more effort was put forth. Diablo 3 seems like a bunch of tile pieces put together inside buildings rather than an artists handcraft. Also the 'toony' aspect of the characters and graphics don't lend themselves well to the dark ominous feel of the game in my opinion. That is subjective --- some may like Diablo 3 graphics better. The graphics look ripped out of warcraft 3 to me and the repetitive animations on the characters like the evil priests look like they could be 5 years or even a decade old. Basically, IMO, some of the atmosphere is lost on the cartoony graphics.
I'm in the middle of ACT2. In Diablo II or in Titan Quest by the middle of Act 2 I was hooked! Line and Sinker. I've played through Diablo II and Titan Quest both Dozens of times - spent hundreds if not thousands of hours on each of those games. Multiple play through on multiple characters and then multiple hard-core runs too! For such a big fan of the genre to walk away from a launch day LAN party of Diablo 3 saying meh --- We'll I hope I'm the only one ---- otherwise this game isn't the gem we've all hoped it was going to be!