I had a very brief conversation with a non-academic friend the other day about the difference in job responsibilities between adjuncts and full-timers (be they tenure-track or lecturers), when he expressed surprise I wasn't paid extra for student advising. Adjuncts are paid strictly by the course and are not expected to do work beyond that. (This is only mostly true: At my institution, they are expected to attend convocation, and one adjunct from each department sits on the faculty senate.) On the other hand, full-time faculty members such as myself are paid to do a range of other things.
Well, what other things? Research is the immediately obvious one, although this expectation varies wildly by institutional type. A Vanderbilt faculty member would be expected to devote the majority of his or her time to research; at my current institution, one annual conference paper is sufficient.
Student advising is another, although some schools farm this out. I have about 50 advisees I am expected to meet with each semester. Some faculty members are involved in other student activities, such as advising a club or overseeing a scholarship competition.
There is committee work. In my case, this means faculty senate. At a non-profit there would be more than this, but at my for-profit, we have less of a pretense to shared governance. (And per another recent conversation with another friend, it is mostly a pretense.)
There is also some service that I would classify as teaching-related - coordinating courses that there are multiple sections of. I do this for our senior leadership and negotiation course, for example.
I'll leave research aside for the moment to focus on teaching-oriented institutions such as my own. I teach four classes a semester; an adjunct who did the same would be paid $56,000 less per year than me. All that advising, teaching service, and committee time is, I assure you, not $56,000 worth of work, no matter how hectic things seem at the height of advising season - despite this tongue-in-cheek assertion that it is.
I teach a course as an adjunct at another institution, by the way, and as a public institution it has a faculty union that is very active in fighting both for full-time and adjunct faculty. I'm in the process of joining that union, even though the rate of pay is nearly irrelevant to me; it strikes me as a basic civic duty, rather like voting or serving on a jury.
One could make an argument that at a research-intensive institution, research is worth a higher per-hour rate of pay, and thus full-time faculty deserve more. (This is certainly arguable, but the point could be made with a straight face, at least.) At a teaching-intensive institution, this is harder to justify. My committee work might deserve a premium, but not one that large.
The problem isn't that I'm overpaid, of course; it's that adjuncts are underpaid.