so let me get this straight about android phones

Brands get the vanilla OS from Google, and change how it looks? Is that all they do? Or can they add features on top of it.

Is there a brand that has features the others don't have? Or should I just go with the Nexus phone, which also seems to be the most future proof (as future proof as one can be in the current revolution of smartphones) since they always get updated and first.

Why should I go with a brand and not, say, the Nexus S for example?