What is the grounds of Thicke's lawsuit, slander?
It definitely sounds close, and "in the same spirit" as Got to Give It Up.
The question is how close/far do the two songs/melodies need to be to justify infringement.
With modern pop music so many different writers, composers, consultants etc etc etc go into making each song into the finished product you hear on the radio, that it's completely feesible to think that somwhere along the way certain melodies and hooks are being lifted from previous works without giving the proper credit.
Most modern pop music along with other genres are really more "creations" than artistry. Everything is so fine tuned and refined, along with mass-adveritised that of course it is going to succeed.
And lets not forget the new trend of pretending new songs/albums have been "leaked" in order to manufacture buzzed ahead of time.