There was an interesting Q & A with Keith Law at this blog
In particular, the portion dealing with GM's was quite interesting, and I believe gives valuable insight. Please forgive me for pasting here.
LIO: With the explosion in fantasy sports, it seems like everyone thinks they could be a GM. Could you give us some insight into the things a GM has to do that the average fan might not know about?
KL: That's a great question. I got an email a few months ago from a reader who said he thought he could do a much better job than (some GM I won't name) if he could get the chance ... and I didn't respond, because I couldn't think of a way of doing it without mincing him into tiny pieces. Seriously - give the average fan a GM job and by 5 pm of his first day, he'll be crying for his mommy.
A GM's job goes so much farther than just setting the major-league roster, but that's the part the average fan thinks about. A GM also has to run the entire baseball operations department, with five or six direct reports including the manager (of course), assistant GM, scouting director, farm director, head trainer, and maybe some special assistants, and to do that job he has to constantly be on top of everything going on with the big-league club and all of his affiliate teams, which includes a lot of crap that you don't hear about on the outside. A GM also has to deal with the media, which even in a soft media market like Toronto is still a big time sink. A GM also has to be the liaison between baseball ops and the rest of the company - marketing, sales, corporate sponsors (all of whom want his time), PR, the team's charitable foundation, and so on. And he has to be accountable to his boss or bosses, which (if he's any good) means managing upwards, regularly talking to or meeting the President or the owner or both.
To be good at the job, a GM also has to have a lot of characteristics other than the ability to make trades and write comments on message boards. He has to be a leader, has to be somewhat articulate (a rule I admit is often broken) to be able to deal with the press and to make a strong impression on people in finance or with corporate sponsors, has to have some financial sense, and should be able to evaluate players, whether it's via stats or scouting or both. He has to be able to think strategically, to craft a long-term plan while dealing with short-term realities, and to ignore the media and fans who demand this move or that. And it doesn't hurt to be just plain smart, because a good GM assimilates information from all kinds of sources, synthesizes it, and adjusts his long-term and short-term plans accordingly. Granted, not all GMs have all these traits, but they all have some of them, even the ones we all ridicule. What we see is when a GM doesn't have good baseball skills, and ultimately that will get him fired because results on the field matter most, but there's a lot more to the job than that.
Anyway, that's just off the top of my head. It is a huge job, with lots of responsibilities and pressures and none of the boundaries of time that a typical office job has - if you're a GM, your phone will sometimes ring at 11 pm, and you have to take it. You're accountable to everyone.