Take it you haven't played Halo CE?  It has a slowly-regenerating energy shield, on top of a health bar.

Halo CE was the first console shooter I ever enjoyed. TBH I think the PS2 was too weak to play the modern Halo type shooters and suffered from poor controls due to low framerates which is probably a major reason why they never really worked for me.

Ehh IMO the halo franchise has been declining, halo 1 was EPIC for its time, halo 2 was good the online made it VERY fun despite its HUGE amount of glitches/not being done, and halo 3 is meh.

The answer to the question is variety of weaponry, combat balance, and the defeat of cheap absurd tactics. Anyone who has played shooters for any length of time is well versed in the alpha principle. One weapon to rule them all, one location to camp from, one way to win the game, and all you need to do is find them.

Every weapon in Halo is balanced with difficulty of use, or imbued with a tactical advantage. That plasma pistol is pretty powerful, but the drawback is if you miss with it your toast. The rocket launcher reload is painfully slow, but a direct hit is lethal. The sword is good, but you have to get real close. A frontal attack is suicide if the other guy has a sticky grenade, and so forth. There is no single best weapon. There is the right weapon for the job, and each requires a different strategy. So your never at the mercy of another player. You always have options even if you do not get the weapon you want.

The maps are well balanced, and thwart choke points. Further more there is no value in camping anything. All weapons being equal, and no safe positions ensure that the game is always one of movement, and allocation. Which thwarts the most sadistic strategy. Taking the lead, and holding up. Forcing your enemy to go through some passage you have zeroed in knowing they have no choice but to attack.

There is no one way to win Halo. The game evolves over time one strategy rises to the top until a strategy to defeat it arises. Then a new strategy must be created. The variation is endless really. That is variation and balance it cannot get stale, because it is always in flux. I could give you endless examples, or the crazy strategies that players have concocted to win. From jamming vehicles into openings to stealing other teams vehicles, and marooning them up in cliffs. Hell why run someone over when you can use a lift and drop a full vehicle right on top of them.

No, actually, that's the only one I've played to a significant degree. I've played Halo 2 a few times, and Halo 3 once, but I've actually beaten Halo CE & played it online quite a few times.

That doesn't really count much, because the "health bar" is only about 10% of a typical health bar. Not the same idea, and not very well done.