A sad turn for a series and a tremendous disappointment
1. Single player. I've never been a big fan of COD single player. But COD1, CODUO, COD2, and COD4 were great in parts and frustrating in others. MW2 single player is completely forgettable. I really miss the lean button as everyone can lean out and shoot at you, but you can't return the favor. If you buy it MW2, skip the single player mode.
2. Multiplayer. Overall it's good if you are new to online competitive multiplayer. Otherwise, here's the problems:
1. No dedicated servers. The host has a huge advantage. If you are the host, you will run around in a killing spree wondering why everyone is so slow to react to you. If you are not the host (most of the time), you will die suddenly and often. You'll have to watch to the kill cam to see what happened. When the lag differences are minimal, this game approaches fun. But all too often, the lag is very significant and completely destroys any fairness or balance. If you are used to seeing an enemy or getting tagged from an unseen enemy and being able to take cover or respond with some intelligent action, expect to be frustrated. Often you see an enemy and a few microseconds later you are dead. If you are shot in the back, you will just die as if you were sniped. You will often take cover only to die inexplicably a few seconds later from seemingly nothing (old age?). You will shoot enemies perfectly and miss. What you are seeing and what is actually happening are often two very different things. To top it off, cheaters with aimbots, etc can run a muck with no admin to kick and ban them.
2. Everything is overpowered. In COD4 a higher skilled player is very likely to win every encounter with a lower skilled player. This is no longer the case. In MW2, it almost doesn't matter what weapon you choose or how you use it, because everything is overpowered and shoots rapidly with no recoil. Assault rifles are the new SMG's. SMG's are pointless except they can be upgraded so that you dual wield one in each hand, making aiming impossible, but no one aims anyway. Knifing ranges have been increased, and there's a dozen new kill streaks that rain unfair death from above. There's run exploits, infinite grenade launcher exploits, etc. Getting kills requires very little if any skill. There's not much you can do to avoid constant death. You can't even lean around corners. The kill streaks in COD4 were annoying at best. MW2 cranks the annoyance up to almost unplayability, especially if you end up on the losing team. When you die, very seldom will you feel that you were beaten by skilled play.
3. The level design is overly complicated and cluttered There's always multiple stories with multiple corners and alleyways. The levels are cluttered with bright colored objects. It's a camping/hiding dream. The clutter makes finding the enemy much harder and all the corners facilitate camping. My eyes tire quickly from trying to recognize enemies from all the clutter. The result is chaotic battles with very little order or predictability.
MW2 transitions the COD series to a wild arcade competition where skill has a much harder time overcoming luck. The result:
I've probably got 600+ hours into COD4. I even put in 150+ hours into COD WW. I've got less than 30 hours into MW2.