It's a joy to work in Celtx on my desktop but much of the functionality that makes Celtx so great is lost on the iPad.
It's frustrating that I can't:
create and email PDFs - this could well be the most important thing that the app is missing.
write (and sync) prose - the lack of this is the only reason that I don't write prose in the desktop program.
access my index cards - I would even be happy to pay for this as a separate mobile app.
Also, I concur with the user who suggested stylus support for markup on the iPad. It would be useful, but not necessary.