TinyMCE is not viable if users will be pasting content from Word documents, and generally does a terrible job of dealing with pasted styles...nearly impossible to override, so you end up pasting into Notepad, etc. etc. At my job, we're going to migrate our pilot Drupal sites from TinyMCe to FCKEditor due to this reason alone: To our users (content editors/admins who were stuck in Contribute before), this makes the Drupal software as a whole look bad. Even with the "Word Paste" plugin installed for TinyMCE, users have to click a special "Word Paste" button. This is silly, in FCKEditor you just paste normally.

One of my least computer-savvy clients for whom I built a Drupal site in the spring had absolutely no trouble using FCKEditor, while even I have a hard time wrangling some content in TinyMCE.

In short, TinyMCE's time has come and gone. It's still a decent RTE, but FCK has definitely surpassed it.