1. As my character levels up he unlocks 'Perks' like Marathon. This means that he's become better conditioned to run, and is more fit than the other guys on the battlefield. He can also unlock other Perks that differentiate him from the other 'characters' of other players (remember how you all like to say how perks in Skyrim are so important to defining your character now that attributes are gone?)
2. My character is what I play online. In Call of Duty I make a class, select a weapon, select perks, select an appearance, etc. just like in Skyrim. I can give him a name (most people name their class 'Sniper', I can name it 'Bob'). Remember how you guys often say that people who complain about Skyrim's lack of RPG mechanics just lack 'imagination' and you need to use a heck of a lot of imagination to RP in Skyrim? This is true for Black Ops too. If all that it takes ti make an RPG is imagination, then I can 'imagine' that my sniper class in Black Ops, named Bob Smith, is a farm boy from Kentucky with a wife at home and two kids, and all he wants to do is survive this [censored] long enough to get home and take his kids to Disneyland like he promised before being deployed overseas. I can even roleplay by being risk adverse, and not running into gunfire, or hanging back in safer corners of the map.
So Bob runs around killing other people for all of 10 minutes, 15-20 if you're playing objective based games, on a very limited amount of maps against other "unique" players with probably about as little knowledge of team tactics as everyone else in the game... yeah, ok, there's really a story there.