As far as the multi player goes, I think WaW is better than 4. For all the reasons Karma said. Limited ammo and all around slower weapons cut down on the run and gun, dogs are better than choppers.. etc. And yea the tanks are easy to take out. If you just run on on them they cant do anything. I have even seen people run up to them and just use them as moving cover until they guy inside become frustrated and just got out to try and gun them down himself.
As far as the setting for the next CoD... I think another near future fictional conflict (like CoD 4) would just be better. Vietnam and Korea sound like good ideas but they usually never pan out to be good games.
Characters for the most parts are just you and the Sarg. everyone else is a no name s.o.b. that you can kill yourself for ammo