In D2 I enjoyed collecting stuff for specific builds, like runewords or specific uniques and to optimize you often used different items per build. In D3 I grew bored because even if I switched to entirely new builds, I wouldnt actually change gear at all, everything hinged in a few select +gooder items (cd, cc, haste mainly). I quit in June and checked it out a couple of times since but only for a day or two usually.
To give you an example of what I liked, the disintegrate spell for wizard had enough cost reduction options that made it viable to cast for extended times with the proper items, that mod was something I specifically sought out for the build. Most other skill mods were +gooder boring (more crit and the like). There were some other items like it for other classes, but imo far too little compared to the huge amount of focus on dps. One glance usually told if something is crap or hot, there was little to no "hmm but it's nice for a xyz build I'll stash that". I guess I enjoy the item hunt more if it occasionally opens entirely new ways to play the classes, instead of just making me kill a second faster. I also liked the addition of paragon but it too felt too flat - afaik they expand on that, good move imo. In D2 you were always at least getting xp even if nothing awesome dropped, endless paragon probably has that effect.
Other issues were less game design-related like the retarded story that's also pushed in your face every step of the way, but afaik that's gone. I'd also like less convoluted systems in place and more straight monster bashing, I hear they build in some stuff I'm not sure about but with just rumors or outdated beta info I dont want to judge early.
TLDR: "I guess I enjoy the item hunt more if it occasionally opens entirely new ways to play the classes, instead of just making me kill a second faster." .. after re-reading this post that line really stuck out as the main point.