It's a freaking invocation. I don't think it's a big deal who he picks.
It's the symbolism. It's someone who represents more of the same old thing that we've seen for years, as opposed to something different that America voted for. It's insulting to gay & lesbians, it's yet another unnecessary and ridiculous "compromise", and it's just plain stupid.
Why are gays angry about Rick Warren when OBAMA DOESN'T SUPPORT GAY MARRIAGE?
Another good point.
And maybe it's just me and my lack of love for symbolism, but I really don't get the fuss over this. He could've picked Louis Farrakahn and I wouldn't dislike him for it (I'd probably criticize the fact that he picked him for the invocation after rejecting his endorsement, but I wouldn't criticize him because Farrakahn is an anti-semite / racist).