Let us consider the sterotypical fantasy landscape. There once was a great and powerful empire that fell. Its strong hold have now become lairs for monsters, the evils it once kept pushed to the borders is moved in between the reamining centers of civilization. What was once a unified whole (or at least 2 or 3 unified wholes) becomes a patchwork of small kingdoms.
Enter adventurers.
A few generations of evil parties taking over kingdoms and trying to forge new empires, falling to infighting or good adventurers. A few kingdoms where adventuring clerics or paladins establish theocracies with or without the will of the people, a few mageocracies here and there, etc. This can go on for centuries, as long as wealth is won and lost and monsters are allowed to breed.
Ultimatelly, however, I think an equilibrium would be reached. An oligarcical meritocracy would place power in the hands of a semi-democratic large body of adventurers under a very simple set of guidelines. Basically the goverment runs on the basis of a huge adventuring party model. Every few years the group meets (or all who wish to do so) and places someone(s) in charge based on their ability to keep the inns open, the roads paved, and everything running smoothly. "Its your turn to stay at home and take care of this stuff so the rest of us don't have to worry about it." That person or persons is the goverment. They stay in line because they know that no matter how powerful they are, or how much support they could get, a significant minority in the group could make things so difficult that seizing ultimate power isn't worth the effort. Overt corruption isn't worth the effort as there are enough good adventurers to disrupt any plans. The good guys can't drive out all the evil adventurers for the same reason. The order of the day is "Leave me alone, I will leave you alone."
Meanwhile the commoners, merchants, and others in the land are kept relativelly safe and the economy is kept flowing. Probablly as happy or happier than under other forms of government. Taxes are, usually, lighter. Most of the cities and farmlands are safer. If their are major disasters, which are rare, good adventurers help restore the area with magic and gold. If people want to be part of the ruling class all they have to do is go out and earn their place through strength of arm, mind, will, or personality.
However, elaborate rules about dealing w/monsterous humanoids and other creatures also develop. They don't destroy them all, they want their to be more in a few years or the system is doomed. Things are allowed to escape for the good of society. Preserves, goblinoid reservations, are established - knowing that they will overbreed and get out and have to be dealt with. Rules of exploration, claim staking, etc are the highest laws in the land. "Poaching" is the worst crime of all. Oh, and wealth is not inherited. It must be earned. But that doesn't mean the children of adventurers don't have a headstart on things.
Sure adventurers want wealth and power. But what is the power of a crown compared to Epic level spells or swords and armor that let you fight dragons single handedly. I think traditional goverment, for the aventurer, becomes a neccesary evil. Something that has to be handled so they can get on with life.
Now, certainly it may not work out like that, but given a significant body of adventurers on a continent I can see them working out either a totally new form of government (one which might be closer to tribal or chiefdom based societies due to the relative power and wealth of the constituants) or coming to a formal or informal arrangement to not take positions of power, being kept in check by tradition and the might of other parties.
