Combat in D&D was a thorn in my side for a long time. I have a confession: I starting gaming as a wargamer. That being the case, I craved a degree of realism in my gaming, or at least an attempt to make sense. Combat in D&D really and truly does neither, at least when you operate under the misconception that D&D combat models combat.
Let's look at it:
1.) All weapons do d6 damage
2.) Armor makes you harder to hit
3.) The notion of hit points
Not only are the points I listed rather ambiguously described (if at all), there are also many things missing from a realistic combat model. Things like variable weapon damage (added in Supplement I), armor should absorb damage rather than make it more difficult to hit the wearer, damage should carry penalties, a one-minute combat round is far too long, and initiative was vague.
Yet, consider this: D&D combat does not model combat. It actually models results. The dice rolls along the way are simply for dramatic tension, it is the result that truly matters. As long as the system arrives at the proper conclusion (ie delivers the proper result based upon the circumstances of the combat), the system did its job. D&D is a game and like any other game it exists to determine the winner of its particular contest. We play Monopoly to drive our loved ones into abject poverty, not to argue over whether the person with the shoe should have a -4 to their roll because they are "walking" around the board.
It is also useful to consider D&D's genesis: it was a miniatures wargame. Delivering a consistent, believable result was Chainmail's goal, as with any wargame. It naturally became D&D's goal as well. I believe this is why combat is one of the most difficult things to house rule in D&D. People are trying to house rule a lack of realism by adding realistic bits and pieces. There isn't a lack of realism, there is an absence of realism. You can't add house rules to something that doesn't exist. If you want more realistic combat in D&D you will have to write it from the ground up, not try to bolt it onto D&D results-modelling combat.
If you wanted to somehow house rule some X factor into the formula that delivers the results, that is more plausible. I'm not entirely sure how, or even why, someone would want to, though. The results delivered by D&D's combat system are consistent, fairly accurate, and above all, realistic. At least as realistic as a fantasy game can be, anyway.