Call of Duty: World at War is not actually loving awful, it's just hilariously average and a play it safe sequel. There is nothing new in it that Call of Duty 4 didn't do better a year before it. Treyarch just can't do originality. Hell even the Quantum of Solace game was a mashup of mechanics from other games that have done it better.
Get Call of Duty 4. It's a much higher quality game than Call of Duty WaW, and the fact that it's not set in WWII is reason enough to prefer it over WaW.