Doom has never really changes (except for the graphics which are MUCH
better than with Doom or Doom 2.
Basically you run around, kill zombies/monsters do little tasks and pick up whatever weapon/ammu you come across...with the harder opponents you find the better weapons.
COD 2 is not really a change from COD 1 except it's a different map but here the same grab whatever gun tickles your fancy...I usually ditch the provided weapon and use whatever material I can muster from the enemies I killed..it's like unlimited ammu
If you're familiar with the MoH series...CoD is pretty similar (personally I like CoD a little better than Moh...but that's just me)