As stated above, the game plays like your standard Diablo
style action-RPG. All of your standard RPG systems are in play. Killing monsters, solving quests, gaining experience, leveling up, getting new items and equipment, it’s all in there. Game control is extremely streamlined and consists of little more than moving a cursor around, clicking where you want to go, clicking what you want to attack, and clicking what you want to interact with. Even the game's equipment window and interface is nearly identical to Diablo
In addition to gaining power purely from leveling up, each character has Diablo and World of Warcraft style skill trees as well. As characters level up, they accrue skill points which are then assigned to whatever special abilities to choose. Certain skills have prerequisites so the same character can be vastly different for different players depending on how you grow them.
There is, however, one major difference between Dungeon Siege II and many games like it. Rather than going it alone with a single character, you can control an entire party when you go into battle. This adds a strategic element to an otherwise fully action based system. You need to make sure your tough warriors are on the front line and your magic users are safe in the back row. You know, typical RPG stuff. If I had to boil it down further, I would say it has the interface and gameplay of Diablo with the party customization and coordinated strategy of a Bioware game like Knights of the Old Republic.