Campaign, that is the only reason I play most shooters.
Why?
Counter Strike,
Battlefield 2,
Day of Defeat: Source,
These games have already been made, nothing released since has had multiplayer good enough to tempt me away for any length of time. Modern Warfare 1 came close, but 2 and Blops blew it. So if the campaign sucks the chances are I'll be finished with a new shooter inside a week, Halo Reach barely lasted a day longer even with the multi player.