I don't think I made that comparison that Natural Selection is similar to Tremulous here. I believe we can all agree gameplay-wise Tremulous excels in bringing in a fresh new experience by combing old elements like RTS, FPS, cooperation and domination to a whole new level, but the whole human vs. alien with buildables concept doesn't seem anything new to me in my opinion.
Gloom, Starcraft, AvP (Without the emphasis on buildables), building elements of TF, etc.