Well, since I've been playing in a campaign using all non-human races for about 6 months (DM'd by demiurge1138, if anyone wants to pester him about it), I figure I should say a few things about this.
The setting is a prehistoric campaign, where dinosaurs roam Gaia, and the two main races are kobolds and troglidytes (sp). There are numerous other race options, although the only mammalian player option is to be a ratkin. Since both main races are evil, the world is a fairly dark, bitter conflict zone, and the campaign has basically adapted to that. What makes it playable is fairly easy to define:
1) The world is extremely well defined. demi put a lot of work into it, and the detail and subplots make suspension of disbelief fairly easy. Every player has a fairly good idea of what is going on in the world, what each race is, how the political/societal/natural mechanics of the world work.
2) Racial mindsets are also well defined, and although none of the races act 'human', they all have several easily relatable aspects to them. Even though humans aren't there as a backdrop, the differences in each race stand out enough to make realistic characterization possible.
3) Most of the playable options are humanoid.
It's hard to say whether we're just playing around with the idea of kobolds and lizardfolk riding around on the back of awakened dinosoars, or actually roleplaying our roles, without the 'wow, that's cool' factor. Either way, we've been doing it for some time, and it's still a ton of fun. The campaign is winding down, and the DM is going to be taking a break, but it's been entertaining to try something entirely different from the core stuff that is so ingrained in our psyches.
Now that I'm going to be taking over DMing once the campaign ends, I have to deal with everyone complaining about how I want them to play standard stuff
