Here's my two cents.
Call of Duty 4 was a brilliant game. World at War was based on the same principal but just wasn't as polished.
Most people bought Modern Warfare 2 because of CoD4's success. Then people began to hate on alot of the stuff in MW2 and have subsequently gone back to CoD4, or the Reach Beta for now.
Treyarch's best bet is to keep it simple and solid. Have customisable killstreaks up to 11 kills, and go back to the perk layout of CoD4 (Which would you prefer, Hardline and Commando, or Jug and 3 frag?) And keep FMJ as an attachment rather than a perk, because I thought that was a smart move. Also, Cold Blooded should stay in. Pistol/Stinger secondaries sounds good too.
They can learn from their mistake with World at War, and learn from IW's many, many mistakes with MW2, to produce a truly brilliant game.
For the record, I don't hate MW2, it's frustrating, but I'm always in a full team party of 4, 6 or 9 so we're always organised and usually outplay the enemy.
If you're reading this far, I thank you