Best part of the game's gotta be the B-17 Flying Fortress stage. Heard my heart pumping.
Call of Duty is a hard game and its expansion upped the challenge even more with tightly controlled ammo distribution and unforgiving A.I. You would find yourself out of ammo frequently if you always get holed up somewhere. The A.I. would keep respawning in strategic flanking spots if you won't advance to the next point, and so it's always hard to keep your lifebar up when most of the time you have to bash through the enemy ranks to stop the spawning. Quick saves and reloads are a staple diet here.
Though the game is fairly short, it's so demanding that it's very rewarding to complete and you would want to award yourself a medal after the ordeal, a satisfying one.
KingLim