The major thing is convincing the publisher(s) that the game is gonna be beautiful, incorporate new, great, elements of play, and have something to garner major sales, and have enough proof in market test groups to prove that it can do well.
Gears of War took 2 important historical FPS traits (Graphics, innovative gameplay) and upped the ante by about 2x (cover system, graphics, decent storyline), convinced MS that it will be the flagship title of Christmas 06 (it was), and got good marketing out there. Because of this, this new IP will end up with around 5m units sold by the time it's over with.
Heck, looking at Blue Dragon in Japan, it did very well for putting it on such a bad system. Again, due to implementation of gameplay and enough hype.
Back from the dead, I'm afraid.
