In Halo 2, you take a certain amount of damage after the shields go down before you die, depending on hit location and the difficulty level. Makes sense, since the Human weaponry's ammunition is designed to pierce armor and most of the Covenant's weapons are much like smaller versions of their capital ship's devastating directed plasma weapons, which rip through UNSC titanium hulls like they were paper. (Not counting the carbine, needler, which fire projectiles, and the Brute shot, which fires HE grenades.)
Makes sense to me.
Thinking about it, HL 2's suit also makes sense within the context of the story.
CoD 2, on the other hand, has a really weird way of doing it. I could understand if there were some sort of plot device associated with the soldier's ability to regenerate himself within a few moments of taking heavy enemy fire...but really now, it takes weeks, months, even years to recover from so much as a single damaging bullet wound.
Granted, a health pack scenario isn't all that realistic either, but it's a hell of a lot more reasonable than inexplicable regenerative capability.