First off i'm not going to compare the AI between HL2 and Far Cry because they are both bad, but Far Cry posed much more of a challenge than anything i saw in HL2 (even in easy on FC). And yes, ALOT of things are scripted in HL2. If there's a table set by a door that was put there to block that door it will block it, but if you put like 4 filing cabinets and 2 tables in front of it they can easily push it over and shoot you (ever see that video the guy made?). Highly scripted it far from bad, but i didn't find much to give HL2 credit for aside from textures and physics. The enviorments were hardly anything new, weapons were cliche (aside from grav gun which is just like picking something up and throwing it), No real strategy to different enemies besides "peek-a-boo" shooting. (get behind something, pop up, shoot, get down, repeat) no enemies required different weaknesses aside from machinery with rockets, The puzzles weren't puzzles, just hitting a switch or stacking something, and it was waaaay to linear.
I PERSONALLY enjoyed far cry 10x more than HL2, but the Source Engine has alot more potential for future multiplayer experiences with how easily moddable it is aside from the not so polished netcode (HL2DM anyone?). Hopefully they can polish up the engine with some patches and we can get some good mods.
Variety is key for a replayable game, and that is something HL2 just doesn't have. Especially since i hated the driving sequences.