I noticed that the enemies aren't always the same in number and don't do the same things during my many deaths playing veteran so that's a good thing for the campaign, 4 didn't do that. It is short, took me 6 hours on regular and now I'm playing veteran. With spec ops though there's a lot of extra content that's not adversarial multiplayer.
The new multiplayer maps are nice, much larger and more complex than 4. Where 4 would have 4 enterable buildings and maybe a roof top or second story window here or there MW2 maps have buildings, doors, windows, rooftops, balconies, etc. out the wazoo.
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Commence to Jigglin'
XBL/PSN/GFWL: Jeevz74