Yes, it's just another zombie massacre game. No other game when L4D came out would, as far as I remember, allow you to have such a great time playing it in coop... Hell, no other coop FPS was as fun.
Yes, no plot, that's beside the point. You don't need plot in a game made to be easily picked up online and replayed over and over. Not anymore than AH needs a plot.
I don't know what you mean by stairwells being a trap.. Pretty much any spot on the maps can be trouble.. If anything, narrow passages like stairwells (alleys excluded, since they can drop from above) are a walk in the park with just one shottie in the group, or even just one sniper on his own if it's narrow enough.
I don't see what the big deal is about the engine. It's old, so what? It still looks good and allows a lot of zombies as well as a larger audience (no need for a brand new box to play it). Half-assed or not, it's a ton of fun.
FPS on console is crap, no matter how you spin it.. Only a mouse and keyboard do it justice.
Valve.. Who cares? Buy what's good, don't buy what's not. It doesnt matter if it's from EA or your neighbour's weekend coading. When I'm having a blast blowing up zombies with a couple of friends, the last thing I care about is whether EA or Jimmy Joe Bob coaded it.