For example, I know (from experience) that <IFRAME> and <SPAN> tags are not supported, but would be interested in what tags are, in fact, implemented.
Also, it is a matter of politeness that we shouldn't rip off other folks' bandwidth when pasting images. A lot of sites prevent it, but using your own image hosting or an actual anonymous hosting site is just polite.
The one I like best (better than Imageshack) is:
Imgur -- allows uploading from your computer or re-hosting from a HTML link or pasting it from your clipboard (they also have a neat Firefox add-in and an almost as neat Chrome add in)
I do not have a definitive list of tags supported by FR. LIke you, I know that IFRAME and SPAN, along with <blink> and <marquee> are not supported. I imagine there are quite a few others as well.
John, Would it be convenient for you to provide such a list for future reference?