One thing that seems to pop up in everyone's comments is reach. That's certainly a big factor in fighting. It's a shame it can't be represented easily in systems. D&D of course suffers from being miniatures based and the assumption of a 5' cube of MINE not YOURS.
If we were to equate D&D squares to real-life, then all opponents would effectively have 8' gaps between them. I'm not sure that accurately represents real combat situations. In MMA, opponents have scoring and time limits and the only thing they lose, is the bout. In these situations they are constantly attacking and backing away. But in all the fights I've been in or witnessed, even those with knives or baseball bats, once it gets up and close and personal, it becomes more of a wrestling match than a feint and dodge battle.
When your life is on the line, you're not going to back up to give your opponent with a sword an advantage. Once you're under his guard, assuming you can get there without dying, you're going to stay there and get stabby.
So to me, at least, D&D is more of an MMA fight than a real fight. Everyone is ducking, dodging, feinting, weaving, attacking, defending... it's very dynamic and interesting, but not very realistic. I would favour an agile person with a small blade who could get into the pocket without significant injury over a greatsword wielder who relied on distance and reach, every day in a real fight.
But then I guess it comes down to what you want out of your game. Fun or reality?