HALO is one FPS that really did introduce many new gameplay elements. It offers all that is said below and the modifications already in development look to refine and improve the "HALO experience" even more!
The game has this ability to give the player a grand sense of awe as you look before a big battle field allowing you to take it on either on foot, warthog (a combat jeep without a roof). The problem is it also started the whole (regenerating health and 2 weapon limit malarky).
The reason why it works here is because you also have limited health that gets damaged after your regenerating shield goes down. So its a dual system that most "regenerating health" games don't include.
Not to mention the 2 weapon limitation works in a game where every weapon is balanced, even some of the better ones have a limited ammo meaning you have to use more wisely.
The tactics involved in how you manage the 2 guns make things interesting.
But at a certain point in the game I was asking myself "what am I doing?" and "why is the game called Halo". Throughout the game it gives you a rather tenuous grasp of what you have to do and the why your doing it is very simple and not very interesting.
Fight aliens, save universe.
In the end the game seems to only include Halo at the end and the explanation behind is quite vague. Storytelling is not Bungies strong point.. or character development for that matter.
"Well of course you don't fight your way all through that, you have to go here or there to get through." This is what, for me, made Halo such a disappointment after Half-Life: just more and more and more of the same thing, and it isn't exciting or interesting, it isn't even adrenaline anymore, it's just tedious.
Nearly every Half Life sequence had a storyline logic to it -- a reason for getting through the bits. Halo you don't get that. I swear they had it done, then the project manager said, 'Hey we need about 3 more hours of game time. Add a bunch of dungeon-crawler mazes and load 'em with more bad guys at every corner.
I do like the fact that the Chief doesn't speak to his fellow marines, it just sort of emphasis how emotionally detached he is and how little their lives mean.