For me, one of the (many!) problems with the story is that they first expect the player to believe that humanity is taking its last stand against an overwhelming foe, yet again, just like we did in Gears 1, and again in Gears 2. Then a battle starts and everyone seems to be having fun killing the enemy. If the situation was anywhere near as bleak as the story wants us to believe, then why are the Gears gang running around shouting, "Nice!", "Whoo-hoo!", etc, and generally having a good time until the skirmish ends? And if the Locusts are such a terrifying foe, then why is it that only a handful of soldiers can take out entire armies by themselves?
That being said, the first two Gears games were great, and apart from the weird Dead Space/Killzone-esque part early in the game where you slowly walk around exploring the ship and setting the mood, what I've played of Gears 3 is a lot of fun, too.