There is a strange element of this system that has come to light in the last year or so, as enough time has passed now to compare how certain classes are affected. Fighters, as it happens, do quite well with the system - but since fighters in general need more to go up levels than clerics, druids, thieves, assassins and even mages and illusionists after a certain level, this works out nicely.
But it really depends on how safe a fighter plays it.
Consider: if you gain 20 x.p. every time you take a point of damage, it follows that the better your armor class, the less damage you'll take, and the less experience you'll get. Proposed: an orc has three chances to attack you using a short sword befpre you're able to kill it (we'll suppose your fighter can take 18 damage and live). Here's your likely gained experience from the orc depending upon your armor class:
That's a significant spread - and interesting since, supposing you can take 18 damage, even with the worst armor class possible you're likely to lose less than half your hit points. The gentle reader should note that in the above table, the fighter is assumed to be fighting alone - and thus is gaining the 20 x.p. for each point of damage, PLUS the addition 20 x.p. bonus points for being present in the fight. Also, it does not take into account the average of 45 x.p. gained from killing the orc, assuming it has hit points equal to one 8-sided hit die. (The reader did read the links carefully, right?)
When you fight alone using my system, you really rack up the experience.
(I should pause and point out that the experience gained from killing an orc in the DMG is something like 14 total).
So, the fighter that plays it safe is going to go up levels a lot slower than the fighter that takes risks. If we add the 45 x.p. average for the orc to the total damage, the fighter in plate mail has to kill 16 orcs in one-to-one combat (on average), while the fighter wearing no armor at all has to kill only 6.
Obviously, this doesn't stop players in my world from rushing to the best armor class they can find. Living longer means they can rack up more damage against enemies before getting cut down, and that's usually what happens. A fighter lasting longer takes awhile to make those 10 x.p. per point hits count, but its safer.
However, here is something else to think about.
Remember, I said that the bonus 20 x.p. was also going to the same fighter in the above example, because that fighter was alone. But what if the fighter isn't alone? What then?
Suppose the fighter is with a party of four - what is the spread on experience then? Well, let's separate out the bonus from the fighter's personal gain and see:
Basically, if you're playing it safe, you're screwing your associates out of experience. The mage or cleric standing behind you gets a lot more knowledge about life and combat watching your limbs get hewn from your body if you're not gussied up in plate armor. Not all that nice, but certainly interesting.
I don't think there's a player alive who'd be willing to trade in his or her better armor just so the tag-alongs can get a bit more experience - thus it begs the question: is this fair?
Damn right it's fair. No one can argue that it isn't hugely more vicariously rewarding to watch people get themselves cut to ribbons than to watch swords bounce off shields and breastplates. That is, assuming that the cleric or the mage behind the fighter are also taking part in the battle (crowds in the stands at gladiatorial shows, not taking part, get no experience - but its a nice afternoon for them). If the cleric is watching the fighter's blood pouring out, this spurs the cleric to greater, more frenzied action (one hopes), compelling the non-fighter character to get into the fight and spare the fighter some of that nasty damage. Overall, the party benefits when the fighter isn't clothed in iron ... and doesn't that bring up some marvelous conceptual ideals where it comes to fantasy imagery?
Turns out, Conan's followers did a lot better than King Arthur's, since Conan was half-naked most of the time. Turns out, the chain link bra worn by Red Sonja was there to bestow benefits on the followers beyond a frenzied hard on!
Unless, of course, you're not buying it. Eh, whatever. If it justifies a naked chick warrior, I'm all for it.