I was looking up info on MW2 and after a while I wondered why anyone wants to buy this game since it seems to be taking steps backwards.  I'll explain why.

First of all, for the people just interested in the competitive multiplayer.  From the details released it seems to be identical to all the gameplay modes that WaW has, and only has one extra game type that MW1 does not.   So no solid benefit to buying it.

New maps. WaW had rubbish maps and weapons, so let's just compare it to CoD 4. The MP in that was amazing, but now there will be better graphics, all new maps, new weapons and new perks. They have not really talked about it much as of yet, but IW know what they are doing.

Co-op, this is a big one.  WaW has co-op, you could play the campaign online with 3 others (4 total) or you could play it locally 2 player.  MW2 creators have decided no campaign co-op at all, no local split-screen co-op, and the only co-op it does is 2 player side missions.  A certain other FPS sequal decided to remove campaign co-op and went with side missions instead (a more impressive 8 player at that) yet was heavily criticised for it.  Gamers made it clear they wanted campaign co-op, yet MW2 will not include it when its predecessor did.   WaW outperforms it.

From a narrative perspective, in many cases it doesn't work, and same for pacing, which is why many games choose a dedicated co-op mode, like Uncharted 2. I did play WaW co-op, and it was quite fun, but the SP was just not as good as CoD 4,  I think you can give a much better experience in SP, because co-op is nearly all about just shooting stuff, which is why creating scenarios just for co-op works much better. There are exceptions in games like Gears of War where you are in a squad, but it often feels tacked on

Nazi zombies.  There is no denying this is a very popular mode that was included with WaW.  The chances of it being included in MW2 seem slim though, aside from no official word from Infinity Ward it will be present this isn't a WW2 setting so they would be vastly out of place.  Even if the game were to feature such a mode, Infinity Ward have made clear co-op is two players only, so it will not match up to WaW.

They haven't fully detailed MP, so they might have a horde mode, but yeah, if you like this mode, then it is a loss

Vehicles.   WaW allowed you to control tanks both in single player and multiplayer, Treyarch were very vocal about this pre-release.  IW however seem a bit quite on the vehicle topic, they have listed a few and stated they will appear in single player, but talk quitely when how fully you control them control comes up, going silent when multiplayer is mentioned.  A good chance these won't make it to multiplayer.  If they don't make it the game is on-par with MW1, and steps backwards from WaW.

I stopped playing WaW online after 3 hours, because of the vehicles. The reason there were no vehicles in 4 is because they don't really work. CoD 3 had vehicles online and nobody would say 4 was a step back. There will be vehicles in SP, but this is primarily an FPS not a vehicle game

Campaign.  Ok it is new, hurray!  Without a competitive co-op though how many times are you gonna play through it before getting bored?  Has been stated it will be 6-10 hours long depending on your skill.   Surely this means it is worth a rent only?

You're not, you're going to play it once and go online, why do you need to play things again. I don't get this. Why can't they just create a great experience to be played once. It is not just about the amount of time you spend in the game, once you tire of PS, play online

So there is my argument.  If you love co-op WaW trumps it heavily.  If you do not care for co-op then the game is pretty much just a CoD4 re-release.  Vehicles looking to be in WaWs favour also so far.

It it is a similar game to CoD 4, but if it ain't broke, why fix it?

So why buy Modern Warfare 2?

I take back all compliments i have ever given you!