Yeah, Dungeon Defenders still hasn't managed to release on PC, though that'll be happening soon enough. Ahead of this version's Wednesday, October 19 release, Trendy Entertainment is giving a shout-out to the Steam-exclusive goodies.
First, there's the Aperture Science Handheld Portal Device, usable by the Huntress class. It allows you to open and use portals as you would expect, although it's restricted by a limited energy reserve. So no, it's likely not going to be as overpowered as it initially looks.
Also included are Team Fortress 2 familiars: the Heavy, the Engineer, the Medic, and the Pyro. This seems like a solid fusion of game properties rather than a quick attempt at pulling in Valve fans, but then again, I do have a fairly high tolerance for that sort of thing.