AbdulAlhazred
Legend
I don't just hand out that second +2--players have to give up one human trait to get it--but my players still play demihumans exclusively. And this includes two average optimizers and one serious power gamer.
So, I'm guessing the player's motivations for playing non-humans then have little or nothing to do with mechanics. They simply WANT to play non-humans. IMHO bribing players to go with the DM's plan for things like this rarely works. Either the player does it and ends up playing a character that may not be what they were really interested in, or they just ignore the bribe and the whole thing doesn't work.
I would say you need STORY based reasons for PCs to play humans. Personally I would advise considering story based reasons. When I say this I don't mean "bartenders spit in the non-human's drinks" kind of thing. I mean make the central prophesy of the campaign relate to a human for instance. If humans really are the most important and central race of the setting then give them prime place. The players can be non-humans, but they will be taking on a different role in the overall story. It won't be an unimportant role or uninteresting role, it just may not be the starring role of the whole campaign arc.
If the campaign is more of a gritty intrigue type of game or something like that there may not be quite the same kind of central role that there could be in a story like say Restore the King or something like that. Still, a human PC could be the one that gets to take over an important organization, act as a key contact with a powerful patron, etc.
It can be tough to pull this off and not make it look like you're just giving the human character too much spotlight, but as long as the other's roles in the story are vital and the players get to scratch what itches them it works fine. They can each shine now and then.