Posted June 20th 2007, updated later
In fighting through a level, you can get new challenges and enjoy new experiences by imposing restrictions on yourself. I do a lot of this. You often have to find new ways of tackling things, and it can make the level fascinating all over again. Here are some prime examples to consider.
No reversion (a.k.a. 'single segment')
No dying, nor reverting to a checkpoint, including being automatically reverted by the game when Keyes dies in The Truth and Reconciliation. In short, no reversion, i.e. time-rewinding. I think this is the same as what people mean when they say "single segment"; they're talking of play which is chronologically continuous in the game world. I do all my fighting like this, and consider it a baseline rule for any 'serious' play - as far as I'm concerned at least. After all, dying is defeat, while choosing to revert is like conceding defeat (cheaply escaping an unfavourable situation or whatever). This time your life is really on the line, and if you get killed it's game over! For me "no reversion" is really more a basic philosophy than something I think of as a restriction. I like to play like it's real.
If you're not used to realism-based play like this you'll soon notice the difference, particularly in a long level once you're deep into it with a lot to lose. There's more tension, and getting through will be much more of an achievement. In addition, with this style of play you're likely to sharpen up your combat skills and become a lot more safety conscious, as you no longer have that time-rewind crutch to rely on. That's a facility I see as being for use in a learning phase only, while you hone your skills to the point where you're properly in control and ready to step up to the true challenge, that of playing for real and getting through in one unbroken adventure. In any game intended to simulate a reality, that's the most natural goal after all.
Limit your weapon use to certain weapons, once they become safely available (or even right away if you want to be stricter, though that could make life rather difficult before you can get hold of them). It's easy to get into a bit of a rut using the same old weapons in the same old places, but with weapon restrictions you'll have a lot of new and interesting experiences as you're forced to tackle things differently.
There's loads of potential with this. Limiting yourself to covie weapons can be interesting for example. Without the range of the pistol or sniper rifle, you generally have to get a lot closer, making things quite a bit harder. Mind you, the plasma pistol does have impressive long-range potential you can exploit, with practice. Some other ideas: forbid yourself the long-distance luxury of a sniper rifle, or the vehicle-destroying power of the rocket launcher (vehicles can be tackled with small-arms fire and grenades). I sometimes limit myself to a feeble AR to really bump up the challenge. I've managed to do AOTCR like that, on a foot-based Legendary kill 'em all run. However, that's really tough, and I'd suggest first trying the combination of AR plus launcher, which you'd probably find more fun while still being hard.
Limit your use of vehicles in some way. You could forbid yourself vehicles entirely, or perhaps just forbid the use of certain vehicles. If you don't fancy having to do a lot of running about, another possibility is to allow yourself vehicles for personal transport only, forbidding passengers and offensive use (weaponry and splattering). I often forbid myself vehicles as I like to focus on foot-based soldiering.
There are various restrictions you could apply relating to health, all of which you should really do in conjunction with "no reversion" to make things properly weighty and to outlaw cheap escapes by quick reversion (i.e. when things look like they're about to go bad).
One is to forbid yourself any medkits. If you do this, you'll need to be far more careful in your battling; and once damaged, you'll need to be careful not to accidentally step over a medkit else that would be game over. At the same time things are a lot more realistic, because there's no longer any magical instant healing for you!
More severe, you could forbid yourself from ever losing a health bar (which includes dying, as that equates with losing all your health bars). This is 'zero health loss'. Not recommended if you lose your temper easily! See the perfection challenge for chat about this idea.
Going beyond health restrictions, you could even try to get through with no reversion and without taking a single hit; even a slight one! I've written about this extreme challenge in my separate article, Zero hit and minimal hit.