I'm not in the least disappointed and I think comparing to COD4 is a mistake.
COD4 is lower res than 720p you're just seeing it upscaled - 720p native should look better.
COD4 looks great but at a cost to get those 60 fps - minimal physics (look the barrels... don't move). Most of COD4 is static and if the gameplay wasn't so sweet it would stick out a mile compared to games like Half Life 2 or Crysis
30fps at 720p with the kind of levels/number of players Insomniac seem to be aiming for is actually at the high end of what PS3 (and 360) can produce currently.
If you're really serious about resolution then I'd suggest PC FPS gaming would be more suited to you. COD4 on PC looks a lot better than PS3/360 due to the higher resolution it supports (still no physics for most objects and very limited linear path through SP).
Finally look at the player count of COD4 on PS3/360 or games like GoW and Halo 3 then look at the player count for RFOM2 - 8 coop, 60 MP - there's no way you could get 60fps for that many players without having to really drop the resolution below 720p and probably lose many effects and post-processing effects - the PS3 (nor the 360 to prevent flames) are that powerful graphically. You'd need a pretty decent PC for that.
Try to be reasonable... its easier than you think...
