I think the issue most people take with it is that the Call of Duty franchise isn't really known for its deep, rich story, and based on the rest of the list, it seems that Guinness's voting pool wasn't really representative of any kind of core gaming population.
But you know what? I'll give the ending of Black Ops a watch, see what all those frat boy voters were so excited about.
Seriously? That is what was voted as the greatest ending of all time? I mean, it was a fine ending to the game's story and all, so it's not like the ending is inherently bad. It's just...not really anything spectacular. Oh, so the protagonist didn't overcome his initial brainwashing after all and ended up assassinating Kennedy. What a twist. And while that's all fine and good for that game's story, the best ending of all time it is not.
As for the ending after the credits...I can't find anything really spectacular about Kennedy, Castro, and Nixon getting ready to fight zombies. It's actually sort of laughably stupid.