My biggest gripe is constant internet connection, it's bullshit, especially since my internet connection has the reliability of as a junkie and only works half the time (especially when my mom works from home). So it's going to be a pain in the balls for me to play.
But I'm probably still going to end up buying the game, I caved in and bought Starcraft 2 (which was kind of a mistake cause I got bored of it after a week and none of my friends play and when they actually do I'm working/busy with school shit). But i plan on making time for at least a few games a week with D3 with my friends.
As for mods... I've never used them for D2 so personally it doesn't bother me that much, it's retarded sure but not really going to affect me in the long run.