The original Killzone was just broken from a control and technical standpoint. I think it is borderline unplayable. I actually wasn't excited for Killzone 2 until after playing the PSP game and seeing that these guys actually are capable of making a good game.
And the scarab battles of Halo 3 were great. I think the high point of Halo 3 was that one battle against two of them when you were on that VTOL. The Gears 2 video, and any game that really does this, just piss me off really. In that Gears 2 video, what you could actually do seemed really limited. It looked like there were maybe a handful of enemies you could fight at a time, then there was this entire poorly convinced illusion that you were fighting in this grand battle. At one point you see Marcus pointing over the edge and shooting at the mindless background drones who had no purpose or interactivity other than hiding the pretty bad looking canyons in that scene. The brumacks were cool, but again that entire sequence wasn't impressive at all to me. I really hope that things like that don't appear in Gears 2 that often.