I would like to see a Plain Text option when creating a thread or reply. When copying excerpts from other sites, it becomes very easy to also paste in any underlying formats or styles, which then start to mess up the look of the text. I think webmail providers have the option to enter emails in Plain Text format for this very reason.
It can be circumvented by first copy-pasting text in another Plain Text environment such as Notepad or something simpler like Google's search field, but it is cumbersome and in some cases parts of the styles remain due to browser-specific or general Windows quirkiness.
At any rate, UBB should be enough. Having both Rich Text and UBB in the same text field is annoying and unnecessary.