No, I don't think Valve has been spoiled by L4D. I just see progression.
HL1 was different than HL2 in my opinion. In the first one, you didn't have to think too hard, just move forward. Jumping puzzles, switches and doors, boss battles have all been done but in the first game it was done with a level of atmosphere that pulled you in. I picked to stay in the tram at the end of the HL1 and after seeing the second ending, I firmly believe that my first choice was the right one.
Playing through HL2 was different for me. Nothing really made me want to keep going in the game. I just wanted to get out of that damn lab/fort in the first one. In the second one I just wanted to shoot myself.
CS,TF,HL and all it's mods was a great game but still nothing in the game required more intelligence that what is needed in HL2. Hell, Counterstike doesn't have a story.
Portal while having a story like HL1 {i.e. Escape the Base!} the level design is made so that even young children can enjoy the game. The game gradually explains to you how to use your gun, it doesn't just dump you in the dungeon and not explain gun, tactics,setting,etc.
L4D is more a like an evolved version of CS. CS was supposed to be team based but I have seen matches where one player would practically win the game while his team-mates die. L4D changes it so that you need teamwork to survive if your human. I would consider L4D to be the more smarter audience than the HL audience.