Just to throw my 2 cents in.
I really would like to echo the sentiments of the previous posters vis a vis style selection. It really is hit or miss (most often miss). Yes, if I author content directly in the editor, then it is quite stable. The area where it really lets down is when you cut and paste from another source. Sometimes selecting a style when the cursor is at the beginning of the sentance will apply the style to the whole paragraph. Sometimes, nothing will happen. Also, highlighting a paragraph and selecting a style will sometimes work, but most often not.
Someone made a great comment that users using Tiny are worried about inputting data, not selecting styles. With that in mind, it would be great if at the page level, a user could select that they would use a particular style. Then each and every <p> or <td> element would default to that style, unless a user wanted to change it. At least for me, I use really only 2 or 3 styles within the content area (the rest being within top nav, footers, and sidebars which users don't need to control directly).
One thing that would be a huge help to Tiny is the ability to define at a global level the doctype declaration that your site supports, then present validation/cleanup functionality based on that selection. Certainly, some users have figured out a way of including xhmtl field validation, but it would probably be better to bring this into the core app rather than a big 250 line declaration in your html.
Also, as users can quite often delete stuff that they shouldn't, it would be great if you could implement a tag that told Tiny where to start and stop. Everything in between Tiny would stream to the applet, everything outside a user wouldn't see (except for preview).
I also agree with a previous poster who mentioned the HTML view. It would be a great idea to allow users to toggle their view inline. The current html pop up window is far too small to be useful (ie you have to manually resize before you can really view the code).
Really big fan of TinyMCE. Hope these comments weren't construed as complaints! Keep up the great work.