I enjoy (not all the time, but some of the time) games that try to very strongly simulate the real world and have asked the same question, as I do enjoy such a simulation. So you're not alone, don't let the 4e/indie crowd convince you "no one wants that." They don't and that's fine, but if you do, read on.
The pace of RL combat is mainly affected by the Clausewitzian principles of:
- Friction - in gaming terms, Initiative
- Fog of War - in gaming terms, Perception
Friction is basically the mass of uncertainty that keeps most folks huddled under cover instead of running around like Quake deathmatchers. It's partially fear of death but a host of other factors that make the RPG paradigm of "doing something every 6 seconds without fail" patently unrealistic. Some games have an initiative system that somewhat simulates part of this effect; for example in Alternity you can lose one or both of your actions in a round if you roll poorly enough. You can also add bits of uncertainty to the rules to simulate friction - like one problem with many games is that movement rate, unlike every other part of a character, is fixed not variable, allowing confident exact maneuver. I often use a "Move check" and if you bork it you just may be exposed an extra round while trying to cross that alley under fire...
Fog of War is basically you not really knowing what is going on (this feeds into friction). In most RPGs you see all opponents all the time; making some kind of Perception check to detect them is by far the exception. But even some time outside playing paintball reveals to us that we are woefully unaware of our surroundings, where the enemy is, and where our allies are at any given time. Ironically one of the best ways of simulating this might be to incorporate computers into gaming, such that only opponents (and allies and terrain features) someone sees themselves would be revealed to them. You'd get a hugely different combat dynamic! Much of the firing in RL combat is basically blind and has less than even the usual minimum level of RPG granularity ("1 in 20") chance of hitting.
Both of these factors conspire to get you the slower move to decision that distinguishes real life combat from RPG combat, despite real combat often being "one hit and you're out of the fight." (Ablative hit point models are an attempt to approach this problem indirectly - by reducing one-shotting they try to get the longer battle times.)
Not many published RPGs deal with this topic successfully. Palladium Games' Recon!, a Vietnam war RPG, made a faithful effort towards it - an example is ambushes specifically, the ambushers vs the ambushees get very large bonuses/penalties to what they are doing to reflect the confluence of these factors. I've played it and this game does a good job of simulating the uncertainty of combat.
[Edit: I was in Half Price Books this weekend and was reading through Blood & Guts, a modern warfare supplement for d20 Modern, and it had distinct sub-rules for those wanting a more realistic combat experience.]
An ex-Marine buddy of mine wrote a couple really good articles (oddly enough, for "The Way, The Truth, & The Dice," the e-zine of the Christian Gamers Guild) about adding real world data-based realism to combat, using GURPS as the proximate rule system. The first article covers hit location/effects and the second covers friction (see page 22 of the PDF). They are worth mining for ideas.