the Jester
Legend
The thing is, you need to trade a lot of realism off or else the game bogs down tremendously. Or if you use too much realism, you break genre conventions.
For instance, a realistic damage system where you determine hit location, then apply penalties for broken bones, etc.; morale or coolness under fire rules for pcs ("oh no, my brave paladin just broke and ran"); realistic movement and facing rules (involving a reworking of initiative so I can't just circle to your rear every round); the differences in effectiveness of a mace vs. a sword against chain mail; the effects of different weights on grappling; encumberance effects of storing your items in different locations; realistic healing rules, including infections, gangrene and so forth; etc.
Note that D&D has experimented with some of these in a variety of ways at various times, but they have often been more trouble than they're worth.
For instance, a realistic damage system where you determine hit location, then apply penalties for broken bones, etc.; morale or coolness under fire rules for pcs ("oh no, my brave paladin just broke and ran"); realistic movement and facing rules (involving a reworking of initiative so I can't just circle to your rear every round); the differences in effectiveness of a mace vs. a sword against chain mail; the effects of different weights on grappling; encumberance effects of storing your items in different locations; realistic healing rules, including infections, gangrene and so forth; etc.
Note that D&D has experimented with some of these in a variety of ways at various times, but they have often been more trouble than they're worth.