Combat

Knock Down
Clubbing people to the leg appears to give a good chance of knocking your opponent down, which will give you the chance to do more hits or run for it if the battle is turning sour.

Mobility
If it hasn't changed, every point below 100% mobility reduces your skills by the appropriate amount. So at 94% mobility, a skill of 67% is actually a skill at 61%. Taking those few minor hits at the beginning of a battle could lead to more crippling wounds as the battle goes on. So drop those 80 lbs of meat before heading into battle; it won't go anywhere.

Block or Dodge?
Blocking uses your weapon/shield skill, so if your block ability is higher than your dodge ability, make use of it.

Counterattack
Counterattack is aggressive way of reaction while combat. You can dodge and make counter at the same time instead of just dodging oppenent's attack. The way of dealing damage is same with usual attack. But be careful, both you and oppenent could be damaged at the same time while you are trying counterattack, so if you got hurt then try not to use counterattack.

Ignore
Ignoring oppenent's move is very dangerous because you would get hurt even when oppenent made the slacky attack. If you don't want to waste your turn NEVER use it.

If you are wearing very heavy armor, and want use bows in close combat you may want to chance ignoring your enemy's attack. Ignore can also replace counterstrike and blocking to prevent droping your weapon.

Position
Facing doesn't seem to affect damage dealt since if they see you, they're already facing you. Attacking when you're undetected (Hiding) means the game won't factor in their dodge skill (Though hiding doesn't seem to work well in the current version). However, if YOU get attacked from a square that you can't see (Darkness or being attacked from behind) you will ALWAYS get hit. In short, NEVER turn your back on an enemy while within melee range.

Multiple Combatants
Being attacked by multiple enemies doesn't seem to increase the chances of being hit, but it does multiply the danger of getting flanked. Each attack will get rolled separately, which can exacerbate the penalty you get from fatigue.

Starting with 3.13-p1, every enemy after the first one that is next to the player will lower the player's combat skills by 10%.

Resources
Most of these came with disclaimers about being outdated, however they may give you a view of what weapons do what

weapon table

armor table

thoth's weapon table

toth's tribes table