First of all - Titan Quest is better.
Second, there's only maybe 20 hours of gameplay in Diablo 3, less if you're looking for unique gameplay. Most of it involves kiting mobs, fenangling a gear scaffold on the auction house, and grinding.
The game is incredibly boring, tedious, and has very little unique content. Click click click click. Time to take my haul to the auction house unless I wanna vendor it all for 500 gold.
I've been playing Warcraft 3 for almost 9 years. I'm not likely to go back to playing Diablo 3 for another week.