Economics is a more apt metaphor than other people are giving credit for, but in the end it really isn't "economics."
One thing that I haven't yet seen mentioned is that having reputation enables you to do more things on the site, such as create tags, edit posts, review answers, vote to close/reopen, etc. Enough reputation unlocks moderation tools. Substantial reputations paves the way to moderator candidacy.
Even though reputation isn't a zero-sum system (hint: neither is money), and that giving reputation doesn't cost reputation (except in the case of bounties), in the end the reputation system is something that is held by users and provided to other users.
In other words, I hold the ability to vote (and hence give you 5/10/15 reputation). You want that reputation. Therefore, you have to provide a service that I deem worthy of my vote. Even though my vote costs me nothing, there exists an implicit social contract among many users that quality should be rewarded, and that lack of quality should be reprimanded or ignored. Therefore, the equivalence is that your post meets my standards of quality for giving you a vote.
As with other economic phenomena, we observe patterns that violate this simplistic rule. Higher reputation users tend to earn popularity votes, in the same way that Nike sells sneakers based on brand recognition and not necessarily quality. New users are often given more leeway, and so receive more "charitable" votes even if their post is not the same quality as an experienced user.
All of this also fails to consider the psychological aspects of reputation and badges, which as Qiaochu Yuan somewhat reductively mentions as gamification. In truth, gamification and economics concepts are more intricately related than many think. This is a discussion I could have at length (and one in which I have a fair bit of practical research experience). But the point remains that the system, as originally designed, was created to be an incentivization scheme, but has morphed into an economy of its own. M.SE is probably ideally suited to observe this (SO main is very large, other sites are too small yet).
In the end, the microeconomics are simple: if you enjoy the site, and want to be considered a more substantial contributor, both through intangible reward/recognition, and realizable privilege (enhanced functionality), then you should aim to provide quality commentary, and reward the same in kind.