When Dungeon Siege 1 came out, it was a whole new way of doing RPGs - the what-you-do-is-what-you-become character development, focus on action, good multiplayer and great graphics for the time made it a niche of its own. Dungeon Siege 2 is very much the same type of game with a few enhancements, which earned it some harsh reviews.
It adds very little to the core gameplay of the original, except in one important area -- story. DS1 was a simple hack-and-slash with a threadbare story. DS2 has an involving tale (very Tolkien-like) that gets you into the game and gives a reason to kill the next wave of endless monsters. In addition to a full story, there is less appearance of linearity this time around, and it feels like a much larger and involved game world with a rich history, warring factions, and various races. It is a lengthy game with quite a bit of good content and more options on customizing your team of characters.