I played the original Diablo for a while, and Diablo 2 for several years on (closed) Battle.net. 6th to hit 99 on US East when Lord of Destruction came out (back then, that actually felt like an accomplishment. Now I just think it was silly to lose hours of sleep over a game... I miss being a teenager ). They were very mindless games; click click click, repetitive areas and bosses, etc., but they were an excellent waste of time. The item system (completely random rares, uniques, sets. etc.) is what made the games fun for me.
The original Dungeon Siege seemed very similar to Diablo 2 (IIRC, the item system was almost identical), except you could run around with a party and use power attacks. Having a party wasn't horrible, but I still missed controlling a single character. Power attacks were definitely a nice feature, but it wasn't enough to keep the game from being completely mindless. As far as I can remember, the backstory was pretty lame compared to Diablo / Diablo II (and that's saying something...), but the boss battles were a bit more interesting.
I'm in the same boat now. I still miss Diablo 2, but I can't stand the botfest and outdated graphics. I've played WoW to death, and I'm tired of the endless treadmill. I'm making due with games like L4D, Fallout 3, and The Witcher right now, just waiting for the next great waste of time to come along. After reading this thread, I'll probably read some reviews and either give Titan Quest or Sacred 2 a try.
Edit: I just watched videos for both Titan Quest and Sacred 2. In both, it really looks like you (mostly) focus on a single target at a time, rather than spamming Nova / Firewall / Whirlwind / Multi-Shot / etc. and taking out an entire horde in a few clicks. Is the combat generally slower-paced than Diablo II, or are there massive AoE spells or abilities later in the game? On the other hand, what's the item system like in each game... is it bland, or can you have powerful items with random stats dropping from time to time?