I'm sure most of you have already played it, but for those who didn't, I'll give you my opinions on why CoD4 is better (from a mutliplayer view only).
Short answer: It doesn't do enough.
It's really nice that Treyarch didn't completely screw up the game when they basically built the game from Call of Duty 4, but there aren't enough features that they added in addition to the setting change. All the modes are back from CoD4 with an addition of one more mode, War, a domination gametype, but then again it's only one more gametype when they could have added more possibilities. The maps are nice and well planned, but have too much of a dark theme, which makes it worse on your eyes to see enemy players.
And the guns. Though I understand it is a beta, the guys are TERRIBLE unbalenced. Thoughout the game you will (trust me) be using a sub machinegun, or semiautomatic rifle with a scope or supressor. Bolt-action rifles and machineguns are useless unless you want challenge points, semiauto rifles and submachine guns can snipe and be close range put with the right additions. Unlike in CoD4, it feels like the game can dominate with 1 tpye of gun. At least in CoD4, all guns felt like they had a use, sniping works well, machine guns were great supressors, assault rifles worked equally well, especially in hardcore mode. In hardcore more in WaW, all you really need to use is a MP40 or semiauto rifle with a scope, and you would dominate the entire game even if you sucked.
The dogs don't kill the balence, and actually fit the game, but in hardcore mode, they are hell. All dogs get you in 2 hits in regular but 1 hit in hardcore, which is terrible when getting swarmed, especially without a machine gun. They add a cool element, but overall feels a bit to gimmicky and like a joke.
Still, it manages to do some things well. The lobby system is more like halo, hitting y to see your friends list, and letting your party set to open so anyone can join. Gore is turn on, and looks pretty cool. And people in last stand can be revived.
WaW will sell well, people will leave to play this game, but I really doubt the staying power this will have. If this came out 1 year earlier, CoD4 would have dominated over this, but when all the level 55 prestige 10ers need another game to play, this will be it.