I'm resurrecting this topic, because:
(1) I'm left-handed.
(2) several points relating to that have already been posted, and the alternative is to start all over again from scratch.
For me, mounted combat is really awkward and unintutive, because I have to "unlearn" what feels natural and fight from the wrong side of the horse. At the speed with which mounted combat happens, I don't have time to think about it, and keep trying to fire to my right and slash to the left, with obviously poor results. Combat on foot is less of an issue, but still feels wrong.
Reversing the player animations and implementing a "reversal" option for all corresponding movements would be far from trivial, but should not be an overwhelming task. Question is, is 10-15% of the market worth the time and effort? Personally, I can ignore the odds of encountering left-handed NPCs, but I'm really struggling with the inability to reverse the player character. Having dabbled both with "sword and board" SCA combat and with archery, everything about combat in the game feels backwards, because for me, it is.
Try a simple task like using a pair of scissors in your left hand, if you don't understand the problem; most people just end up crinkling the paper, because the natural movement of the fingers normally keeps the blades together, but in the opposite hand they tend to seperate.
Incidentally, one of the Scottish clans had such a high proportion of left-handers that they built their keeps with reverse spiral stairs. That way, at least they'd end up on even terms with the attackers, not at a disadvantage. Normally, a left-handed attacker was able to negate a defender's advantage in a siege, being able to fight normally around the corners which a right-hander had to fight back-handed against.
The sketchy information on the subject seems to indicate that at least 5-10% of the population was naturally left-handed at that time, although the incidence may have risen to 10-15% in recent years. Periods of high stress seem to lead to increased left-handers in a population group, as agents of change: nature's way of "rolling the dice" for better or for worse when "more of the same" isn't working. For a stagnant heirarchy (like Europe had in the XVI and XVII centuries), change was something to be feared, so left-handers were driven into "conformity", and regarded as "evil". Originally, "sinister" simply referred to "left side", and had no (or at least much more limited) "evil" connotations until later.