My vote for *.docx support.
Docx is the publishing industry's de facto standard, and therefor writers use it as well. (That should be enough really, but oh well.)
Most tutorials on how to submit and format a book for publishing stress using word's latest default format because that's what publishers are used to for the most part, and if they use something else they are accustomed to converting from word to whatever.
Self publishing services also widely use word. (Lulu.com for example.) And while they may use others, they all include word.
I'm content with the filtered web page method (save as filtered webpage then import) but support for Word's flagship format, and arguably the most popular format on the planet as judged by numbers of professional users is hardly trivial and I think should be seriously reconsidered.
Personally, the only reason I updated to the latest version today (12/20/2011) was because I thought (full?) docx support had been added.
I'd like to be able to proofread final drafts from my ereader, and to use my ereader as a sort of on my person offsite backup in case my computer explodes or my house burns down. Support for docx would allow me to put my draft on my device more easily, and thus carry it with me. It would also show me what my book will look like on an ereader, which is growing more and more important from a publishing standpoint.
This is an old subject and covered elsewhere, but absurd anti-necroposting sentiment forces me to rehash as if it were new instead of simply replying to old. I've never found it useful to duplicate effort (which by definition all anti-necroposting policies require).
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