There's something you need to know about the call of Duty series. They have 2 developers working on different games. Infinity ward, Developer of the great COD1, COD2, and COD4 then you have Treyarch, the Developer of COD3 COD2:big red one, Tony Hawk Pro skater, 2-3 spiderman games and Minority Report...
COD: World at War is developed by the Treyarch... I'm not saying it's going to suck, but if history tells me anything, I can't see it being as good as COD4. I'm waiting for COD6 being developed by Infinity Ward
Online coop does make every game better though