Couple quick thoughts in between rounds. This is a mixture of tips and observations.
1. If you didn't like MW1 then you most likely will not like MW2. Nuff said. Zeppo - if you tired of MW1 quickly then MW2 is not for you unfortunately even though I think you'd enjoy it if you gave it a chance. Sure you're going to get rocked in the MP but after a few days you'll be running rings around the 15 yo's:):):)
2. As far as I can tell the game is extremely well balanced and so fears about some weapons or equipment being overpowered aren't justified. Some people aren't big fans of the heartbeat sensor as at first it looks like it'll just tell you where everybody is. The funny thing about COD though is that the pace is so fast that simply knowing where a guy is every three seconds sometimes isn't enough. People are running around so quickly that you can easily get distracted chasing a guy down on the sensor and not pick up the two guys coming up behind you. Also COD is all about how you approach corners and how quickly and how accurately you can get your shots off. Even if you know where a guy is it still comes down to how quickly either of you can fire. I've had a few instances where I'll come around a corner fully expecting to have the jump on someone and then BLAM they upload a shotgun blast of sniper rifle shot and I'm dead. Also you can negate the sensor by using Ninja (over Commando, Steady Aim, Scrambler, SitRep - which detects explosives and tactical insertions, and Last Stand). So you don't give up much to be invisible. Ninja Pro eliminates the sound of your footsteps too which is nice.
3. MW2 MP is very much a Rock, Paper, Scissors style game. If you are having trouble dealing with a certain class then you can always defend against it but then you leave yourself open against or you lose something else.
4. IMO the higher levels do have a slight advantage over the rest of the field. That being said I'm pretty sure the game doesn't throw level 1's in with level 55's when you're searching for games. I might be wrong but it seems like everyone I play against is close to the same level and I'd certainly expect to see a few more 1 - 10's this early on.
5. There are 15 MP maps. 15!!!!!!!! You can also bet we'll be seeing a few more in the next couple months. That is pretty staggering IMO.
6. The addition of call signs (e.g. Titles and Emblems) along with customizable kill streaks and death streaks is nothing short of genius. It seems like you're always unlocking something new to go check out and play around with. It is so addictive:)
7. My current fave load out is a SCAR with Bling Pro (Heartbeat sensor and Red Dot Scope) , Stopping Power Pro and Scrambler Pro. Martydom as my death streak and UAV, Sentry Turret and Harriers as my kill streaks. I rarely hit 7 kills in a row so anything above that would be pretty much out of reach (especially the 25 kill Nuclear Strike, still haven't seen that yet). I'm having a great deal of success with this although some maps (e.g. Afghan) I'm still struggling with.
8. The riot shield is great. Throwing a sticky semtex onto someones shield while they fearlessly charge at you is probably the most rewarding feeling in the game

Playing with the shield is also a ton of fun if the rest of your team follows you appropriately. It is great for flushing out people from hiding spots and then having your wingman take 'em out. You also great experience for that ("Distraction" points).
9. There is only one speed in MW2 and that is full throttle, pedal to the metal. If you aren't frenetically running from spot to spot or sitting on the edge of you seat while hanging out in a sniper spot then you probably aren't doing very well. It is tough for me to play this game casually and not get wrapped up into trying to get as many kills as possible. I think some people really don't enjoy that but for those that do this game can't be beaten.
OK enough rambling for now. Loving this game...