The final battle was a bit too arcadey for my tastes. Oddly enough Ep 1 seemed to me to have better design in general, though the plot and environments of Ep 2 were better.
TF2 has good art direction but not really my sort of thing. Portal though... very well done. It'd be fun to do a "mage" sort of game with the Source engine. Gravity gun and portal spells as the only two weapons. Use the cinematic physics to make buildings and such destructable. A multiplayer game sort of like Magic Carpet:
Claim areas for mana which is used to fuel the spells.
Anyway just imagine uprooting a tree and tossing it at someone through a portal
Ideally fluid physics and speedtree would be nice features. (opening a portal in a river and seeing it pour through)