all games are not created equal

LBP is much more innovative that L4D.  While L4D is a perfect version of a zombie game for me, LBP is so much more than the platformer game on the surface.

L4D is exactly what you see;  a zombie game.  You shoot zombies, then repeat until the game is over.  LBP has a whole layer of creation where you can make whatever you like.  You could even make a zombie level if you wanted to.