Well you seem to buy all your MP games on the 360 so going by exclusives only, I'll say:
Warhawk - At first I didn't like it, but I'm getting the hang of it lately, I saw it once for $40 brand new with the headset at Target.
NGS - Enjoyed the demo, am waiting for a price drop before I get it, it might not be exclusive enough for you if you played the NG or NGB though.
UT3 - Rented it, but might actually buy it once I finish my backlog of games
Hot Shots Golf 5 - Yeah I know it's a golf game, I hate golf, but love this game, go figure
MLB 08 The Show - If you like baseball, this is the best baseball game I've played since that EA one on PS2
GT5P - Personally after some long hard thinking, I'm gonna wait for GT5, but this game is great as it is and is getting a patch soon for damage and better online community
Resistance - Enjoyed the demo, am waiting for a price drop though.
Singstar - If you like singing.
And I also disliked the Uncharted demo, but since people made such a fuzz about it, I went ahead and bought after I got a 20 dollar gift card, finished the game 3 times and am going for a 3rd soon enough to get my last medal and finish it on Crushing, it's easily the best looking console game out there right now and one of the best games out right now.
I enjoyed the Ratchet demo, but I'll wait to buy it at 30 or a discount price.
Also give Heavenly Sword a rent, I haven't played it yet and the demo was rather short, but heard it was a sweet game though.