I was thinking about this in the context of my teaching. In general teachers complain about their students three times as much as students complain about their teachers. Here's an example, take a look at the Chronicle of Higher Education's forum on teaching:
http://chronicle.com/forums/index.php?board=25.0
The overwhelming majority of the activity there consists of complaints about students, with "the thread of teaching despair" currently at 447 pages. There are even entire websites devoted to the practice, like College Misery.
It's not like I've never vented about my job, but I've also always been a bit uneasy with the practice. After reflecting a bit, I think it's because time focused on what other people are doing wrong isn't spent figuring out what you should be doing right.
So as an experiment I'm going to force myself not to complain about any aspect of my job during the coming semester. If I'm happier and better at my craft by the end I'll adopt it as a long-term practice.