I think a plugin that whould do the following whould be great allready:

"Paste and crypt" paste text into field or mark it. hit encrypt. pop/menu/whatever lets you choose or import public keys.

"paste and decrypt" basically the same funtionality backwards

Do you mean a browser plugin? Probably the biggest issue currently is that browser plugins tend to be written in JS. See this for why it's a problem:

http://www.matasano.com/articles/javascript-cryptography/

If we someday have an ecosystem of C browser extensions, in-browser crypto may be much more promising. Or not. I suppose there could be other problems besides the language.