EDIT: Whoopsie, forgot about the Gaming forum.

Alright, here we go:
What I liked:
Plot had some pretty good moments. The boat chase, the fist fight with Shepherd and Soap pulling the knife out of himself, the gulag, the oil platform, "no russian", and the first mission with Soap and Roach were all fantastic. Actually, the whole Afghanistan sequence at the end was pretty cool. It was awesome seeing Price and Nikolai again, and I love that it's open for a sequel.
The various movie references were interesting too. Like Dekker said, I definitely saw references to The Rock in the gulag mission, Goldeneye reference in the first mission with Roach, and the obvious Red Dawn reference in the first mission in Virginia.
Some of the new weapons were pretty awesome, like the remote control for the Predator and the riot shield.
What I didn't like:
Despite Price, Nikolai, Soap, and a few, few others (I got rather attached to Corporal Dunn in particular), you don't really care about the characters as much as you did in MW1. I actually got extremely upset at the end of the original when Zakhaev kills Gaz and Griggs, which I didn't really feel with Allen, Roach, Ghost, or any of the other 141 members that die along the way (in the festeva level or in the mission where Shepherd betrays you).
I didn't really feel like I was losing anything at the end of the DC evacuation scene either...I mean yeah, I was about to be overwhelmed by Russians, but I didn't feel as upset about dying or any real emotional connection by that point to the other guys in the squad like I did in the nuclear sequence in the original. Afterwards, though, I got a bit more attached to Foley and Dunn, and really hope to see them in the sequel.
I don't play for multi, so I can't accurately criticize it.
My take on the story:
General Shepherd was behind everything in the story.
The Russians never did crack the ACS system. He simply used that as a cover story when he disabled the system and allowed the Russians to invade the eastern seaboard, as a casus belli for invading Russia. Since Makarov works apparently as a mercenary terrorist, my thought is that Shepherd hired him to conduct the massacre at the airport, then planted Allen in there and exposed him to Makarov so both of them could get off scot-free with the slaughter.
The intel Roach and Ghost recover near the end probably had the evidence of Shepherd's cooperation with Makarov, which is why Shepherd killed the both of them there. Also explains why Makarov knew where Shepherd was operating from.
Motive: I thought it was dead obvious at the end. Shepherd was orchestrating a war with Russia via Makarov and the 75th (your unit in Virginia) in order to save his reputation and bring some personal closure to what he perceives as his failure after the nuclear attack in Baghdad in the original. With the Secretary of Defense's "blank check" to run a war with terrorism and Russia, now he has the power to wipe out everyone responsible for the nuclear event--ultranationalists and terrorists alike. 141 thwarted that, though.