Call of Duty 2
The best change to the formula from the first full sequel was switching to a heal-over-time approach instead of littering health packs everywhere. It's just as unrealistic (and even seems a bit more plausible if you don't think about it too much), and works better for the kind of gameplay they're going for, where you have to stay in cover, make your shots, and then move on. If you charge the enemy, you're probably going to die, although it's fun to try.
The level design is also a little different. Often you're given multiple objectives at once, and you can tackle them however you want. Usually, you're clearing out buildings, which is fun for a while, but sometimes I would have preferred a little more variety like in the first game. There's enough interesting set pieces to keep you going, and it rarely strays into the annoying territory where it puts you in a weakly fortified position and sends endless waves of enemies at you until the cavalry comes (although it does still happen). It's still fun, although I wonder how much longer it will be.
