Hallways and aliens.
It's fun, but gets boring quick; similar to how Bioshock got boring quickly for me.
Remembering that I'm not so sure I want Blue Shift...
I remember getting to the point of opposing force when Gordon teleports to Xen, and I thoguht "Yeah! No more hallways! Xen+end of game time!" Every chapter after that I thought "It's been long enough since the Xen part, this one has the final boss..."