I'll start by saying I only play Hardcore. It's the only way to play CoD, all of your shots count.
WaW was a success but it was just average. It didn't hold a candle to CoD4. In multiplayer the spawns were shitty, people would spawn behind you, you's regularly spawn infront of enemies, you'd spawn under artillery etc.
The special grenades were useless unlike the very useful flash and stun. last stand made the bolt actions useless. It was so good that everyone had it. Unless you hit a head shot everytime (or were far away with a scope,) you were looking at a 1:1 K to D. at best. Plus the maps weren't great (although none were as bad as Wasteland on MW2).
Once they removed tanks from hardcore I went straight back to CoD4. It was the only thing that seperated it from a poor mans CoD4. You were left with maps designed on a scale for vehicles, just without the vehicles. You'd respawn in the arse end of nowhere then have to trudge your way back to where the action is hoping not to get picked off by a sniper (and because of this flaw there were lots of snipers). The tanks made certain areas of the map really risky for infantry, funneling the action into smaller areas. Take the tanks away and thats gone. It also removed any need to use rockets or satchel charges, tripling the amount of betties over night.
I went spent about 3-4 months on WaW then went back to CoD4. Don't get me started on Nazi Zombies. I played it a few times then got bored of it and never played it again. The only way I found superior was the stat tracking on the WaW website.
WaW was average, MW2 is good but CoD4 was the dogs bollocks.