Uhura is an extremely smart and attractive woman (both versions) so why wouldn't Spock be attracted? Liking Human women is in his genes.
That's not what the original Spock would have done, also don't forget Spock was betrothed. In the original series if Uhura made a pass at him Spock would let her down easily, but he would let her down. Spock grew and developed as he grew older and became less reluctant to show his feelings, but I think it all happened too quickly in this movie, not that I object to the couple, it just does not seem in character for Spock, that's all.