The game's pre-written characters are interesting for the most part when taken on their own (although it's hard not to cringe at the absurd levels of cleavage on display) but the problem is that they are pre-written: you feel like you're simply acting out somebody else's lines. When I play Mass Effect I'm my Shepard, but here in Dungeon Siege III I'm just Reinhart, a pretty snappy wizard whose personality is… well, he's a snappy wizard. This is hardly unusual for the "action RPG" genre: Dungeon Siege as a series bears a lot of mechanical similarities to, say, Diablo, except that in Diablo you're just "the sorcerer" and everybody expects you to burst into flames rather than conversation.
The counter-argument to that is of course "isn't it better to have some RPG elements than none at all?" For my money, no: while it's true that in Diablo your characters have no personality at all, nobody expects them to and nobody complains when they don't. Here in Dungeon Siege III the personalities of the characters feel tacked-on, the conversation choices feel irrelevant, and the inter-party dynamics lack the richness of Obsidian's other efforts. In many ways Dungeon Siege III is the opposite of Obisidian's efforts in Alpha Protocol, where the mechanics and execution were sloppy, but the roleplaying elements were layered and complex. Here the shallow roleplaying ontop of a deep series of combat mechanics just feels false and unnecessary, which is in many ways worse than feeling nothing at all.