Halo's difficulty selection is just like Goldeneye/Perfect Dark's, except that obviously you aren't required to complete extra objectives to complete in the ascending difficulty levels. Great system.
If you're finding things too taxing in a level in a given difficulty, you just notch it down and continue on a lower setting. However, you can only finish levels in difficulty classes that have been completed in all prior levels up to that point, which makes for a subtle egging-on of the player to finish the entire game in the more difficult settings.
...hmmm...a Bungie/Rare collaboration would be sweet sometime down the road...