The specialness is that YOU feel as if you're the alien. Many of the "aliens" in this game are actually extinct creatures native to the planet earth. Even the real aliens are far longer on earth than humans, and seem to know the planet better than the humans do, they seem to be more "native" to it.
The feeling in some parts is as if the earth itself is against you. Revolt of nature so to speak. In UFO defense you're clearly fighting alien invaders from outer space, the feeling is vastly different in TFTD.
Edited by itworks, 09 March 2010 - 11:24 AM.