Alright, so here's another one that really pisses me off.

Artists who not only watermark their works, but make the watermark so damned dominant over the image that you can't see the detail and intricacy of their work. I mean seriously, you are posting your work on the internet people. Get over yourselves. If it's on the net, and it's good, someone is going to use it for their own stuff. Wall papers, textures in games, phone screen savers, whatever. If your work is good enough that people want to use it, then take it as a compliment. If they make a profit off from it, then do something about it. Blocking out good work with a watermark is like walking up to every painting in the Louvre and using a can of spray paint to write the artists name over the top of it and cover up 30% of the image. Would YOU go to a museum that did that?

Get over yourselves. Either don't post on the net, or quit worrying that the work you aren't even selling might be enjoyed by someone enough for them to want to use it.