Disclaimer: I have only played Dungeon Siege 1 and Torchlight, so I can't comment on Diablo or Dungeon Siege 2 and 3, but to say that Dungeon Siege 1 is a better game than Torchlight? I'd say they are more like apple and orange: one is more about party-based while the other is more about a single character (unless you count your pet as part of a party). Both are fun, but I would not exactly call Dungeon Siege 1 a better game than Torchlight.
They're still in the same sub-genre and the core mechanics are very similar. I wouldn't call them apples and oranges. More like red and green apples. It's not much of a party in Dungeon Siege either, you may have more followers but all they do is charge forward and kill everything like in every other hack'n'slash / action RPG.
As for Dungeon Siege 3 I enjoyed it a lot more than I thought I would and yes, I hope they find new ways than potion chugging, that's neither a fun or interesting mechanic especially when you have to wear like a 100 when you are going into a dungeon like in Inquisitor.
The system in Path of Exile sounds quite interesting, luckily I still had my beta account working so I'm going to give that another shot.