I really, really, really wish that folks who release WordPress themes and plugins would include useful screenshots that actually show what the theme looks like or what the plugin actually does. At the very least, it would be nice if they took the time to write a detailed description of the plugin. I always skip themes and plugins that make _me_ have to work to figure out what they look like or do.
On that note, themes.wordpress.com has bugged me for quite some time with the thumbnails that don’t work!
I think that comes down to lazy theme authors who don’t upload their screenshots when they upload their themes. And yes, it bugs me, too. They only hurt themselves, though, because I don’t even bother with the test runs for those themes, preferring instead the themes that I can see at a glance. I just don’t have enough time in my day to preview every single theme that doesn’t have a screenshot.