The menus. The complications. The convoluted menus. The mind-blowing tutorial. Oh, and there's a very good tower defense game in here, too.
Dungeon Defenders is really a simple, enjoyable game. It's tower defense. Enemies spawn at a few points, walk a set path, and attack a crystal at the end of the path. The heroes in the game summon towers and fight back the orcs, goblins, and trolls, defending the crystals from destruction. It's a standard idea with up to four simultaneous players all working together to defend the crystals from all attacks.