I have already been playing it on the PS3.

I somewhat disagree with the idea that it could have been an expansion.
While it looks and somewhat feels like past CoDs, it also has new maps, some new weapons, new killstreak loadouts, etc.
It also has a bad spawn system.

It definitely could have been just an expansion for MW2. Not necessarily for other CoD's in the series, but it's just MW2 with new maps.. maybe a new texture here and there but it certainly does have the same bad spawn system that MW2 had as you mentioned. It really depends on your game mode though. I noticed playing something other than TDM eased up the spawn problem

The game has been on the same game engine for years. They're trying to keep up with the destructible environment that BF3 has injected into the gaming market, but it ends up being them stretching the limits of the same old CoD engine. They need to start from the ground up and just make a new engine. Which is prolly what we'll see next time around. Hopefully.

I do like where they're taking the the abilities for the weapons... like being able to add the "kick" perk to your Barrett will help alleviate the weapons "kick". There's definitely some good elements to the game and they've added some stuff to keep it different... but when you consider Diablo 2 "Lords of Destruction" was just a $20 expansion pack to the original game... and the same with Starcraft "Brood War", you got added features to make the game streamlined and user friendly.. added maps. added story lines.... it just makes me think that there was a time when you got everything in an expansion pack for another $20 almost a year later.. now they're charging you for a whole game at $60 unless you get it on sale. I know they need to make their money, but I can't help but feel like I'm getting a re-hash of the old stuff when it used to be free from modders or $20 from the devs.

This free weekend will only be the multiplayer portion of the game, so players will get a small look at the game. I liked the single player myself. If I had to pick at something that has peeved me a bit as the CoD series has moved along, it's the progression toward being "arcade like". Every time I turn around in the game.. I'm getting a prize for this, an unlocked something or other, a new perk. I feel like I'm getting something all the time similar to an arcade like feeling. I fell in love with CoD in the first place because it was a fun shooter. It didn't matter how many perks, weapons, prizes, packages, or unlocks I got. The game was just fun to play. Now I feel like you can't even run in the game without getting some award. You even get rewarded for dying. Rewards for doing poorly... so EVERYONE feels like they're doing good and contributing to the battle.