Modern Warfare 2 (COD 4) is not slow paced, but it is fairly easy on the lowest difficulty and I really liked the game a lot. The sequels went progressively downhill.
I also enjoyed Borderlands. It is easy if you level sufficiently, but very difficult if you are under-levelled. If you are having difficulty, just do more side missions to level up.
Half Life 2 I dont recall being that easy, especially the final battle where you are sort of swarmed by striders. Half Life one was probably easier. Both games were fun though, with more puzzles than the normal shooter if you like that kind of thing.
And I thought Fallout 3 was just plain difficult on the easiest setting. Weak guns and never seemed to have enough ammo. Maybe I just did not get the hang of it or was missing something.