The mistake that SNL often makes -- and it's always less funny this way -- is to try too hard to nail a character. In other words, they sometimes slip into the trap of doing imitations rather than impressions. I would argue that this is also a broader issue with their sketches (just copying rather than satirizing -- there's a difference), but that's a post for another time and place.
For example, Armisen's Obama was too much of a forced imitation and not enough of an impression. The same with Hammond's McCain. But compare those to Jason Sudeikis' Joe Biden, which is an exaggeration -- a satire -- of Biden and, consequently, is much funnier than a carbon-copy imitation would be. It's the Chevy Chase as Ford Theory. Chase never bothered to wear makeup and he never tried to nail the Ford voice, but he defined President Ford. And it became a legendary character. [More after the jump...]
Another example: Will Ferrell's George W. Bush and Dana Carvey's Bush 41 are both impressions rather than straight imitations. Both were hilarious, and both have held up over time -- they're just as funny today as they were eight or 20 years ago. They were also effortless. Ferrell and Carvey literally expanded their impressions into what practically became stand-alone characters, as Chevy Chase did with Ford.By shedding the pretense of nailing the look or the voice, those guys could emerse themselves in the characters and have fun with mannerisms and quirks that neither of the actual Bush presidents ever had. Carvey's spastic Bush 41 laugh and "nahhh-gahhh-dahh," for example. In fact, the real Bush 41 appeared on SNL and joked that he'd never once said, "Naahh-gah-dah." Which is true. Nevertheless, we'll always associate the real Bush 41 with "nah-gah-dah."So it's not so important to find an actor who looks and sounds exactly like President-elect Obama. Instead, it seems to me that the hook is to find someone who's naturally funny, talented and charming -- first and foremost -- and who can do something with the character. In other words, there are a crapload of hoopleheads on YouTube who can do spot-on celebrity imitations, but they're also mostly unfunny and, frankly, kinda creepy to watch.