I always play the campaign mode, so, no campaign = me not buying (unless it's genuinely dirt cheap, like the skinflint MF that I am)
Seriously, though, I can't get myself to MMO it that often because the lack of genuine co-op-ing that pervades servers is annoying. I tend to LAN it with around eight to twelve other guys (plus the occasional girl) as it's more fun, and leads to much less rage-quitting and much more friendly insults, as opposed to heartfelt insults which other players won't even hear let alone acknowledge.
This means more offline play, naturally, so a strong single-player campaign (e.g. CoD-MW which was awesome in both areas) Consequently, I will not and never will get Black Ops.