As a developer, all I ask for is a description of what happened, what you expected, and details on your configuration (OS, browser, hardware). It would be even more awesome if you could provide a set of steps to reproduce it, but that's not necessary.

But honestly, a bad error report is usually more useful than no error report. I'll probably disregard it if it doesn't have much info, but if I see a lot of similar reports, I can glean info from them to get an idea of what went wrong. But if you have the above, that can mean the difference between a fix being done really soon or me needing to wait for more info.