Well, I don't mean to pick on one of your favorite games...just a difference of opinion. I just remember being *engaged* with Half-Life 2, something that isn't happening here. "Survive by getting to the surface" is just another way of saying "follow the level design", which most shooters do, but with no backing narrative structure to keep me engaged, it's tough...especially when the environments and enemies are not particularly varied. If there were puzzles other than the meat crane, they were so mentally unchallenging that I didn't even think of them as puzzles.
I'm sure an issue is also because I played this in reverse...all of these enemies, save for the Vorts, are completely old to me because I've played all the HL2 stuff. Not a fair criticism there, but...(shrug)