Multilayer compared to the modern warfare sub-series?
Better? Worse? Completely incomparable?
Of the two companies to make the CoD series I've always felt that Infinity Ward made a superior game to that of Treyarch
Largely the usual CoD affair with a few tweaks:
Much more customisation. When you gain ranks it literally opens up a ton of options. You can even attach a flamethrower to your weapon. Only drawback is you need CoD points, so save them up to buy what you really need.
Less stupidity than MW2. The RC killstreak award is about as silly as it gets, no nukes, dual shotguns, ect.
Better killstreak awards. Not only can you get a chopper, but at the cost of a few more kills you can actually pilot one.
Zombies. If that's your thing...
More maps. Yup, no Activision "lololol dlc is how we make money".
Better map selection system. No veto this time, you get to vote between the choice of 2 maps. No more vetoing a bad one to find the next sucks worse.
Other than that it's nicely polished, well balanced, and enjoyable. I know Treyarch are only working on the system IW created, but they've largely improved it in every way.