Your comments

Pshaw! The icon is very professional. It looks like a physical representation of a T key form the soft keyboard and is representative of the app's function. Don't change it into yet another document with pen cliché piece. If you like it, and only one person doesn't, then there's nothing to see here. Move along. :)
You asked, so here it is:

I would like to see a couple options for applying CSS to the preview, say using sans-serif fonts or serif fonts, different families, etc. The app MarkdownNote does this very well. This is a 'must have' feature. The standard webkit HTML styles are painful to look at for very long.

Another option I'd like is live preview, ala MarkdownNote app. It's nice to have a side-by-side like that, but it's not necessary. Label this a 'nice to have' feature.

I do make good use of Markdown Extras, so if you could ever implement those in the preview you'd be a Hero. I don't think anyone has managed that yet on the iPad. This is another 'nice to have' for the vast majority of users, I'm sure.

I would like syntax highlighting. I implemented that in TextMate to make my life easier. It's certainly not needed, but my eyes prefer a difference in color for, for lack of a better way to say it, formatting vs text. This is again a 'nice to have' feature for the vast majority of users. Some may not even want it at all, so it should be a toggle item if you ever do implement it.

How do I use Markdown? Well, I use it all over the place. I use it for my blog posts, for developer notes, task descriptions, and so much more. On my company blog, we have implemented a Markdown editor with live preview in PHP, so I mostly stick to using our built page, but for my personal blog, our developer/support site, and a few other places I use Markdown, I use an editor to type it al up and preview it, then copy/paste into the appropriate web form where it gets parsed on view.

There is one instance where being able to FTP the file straight from Textastic would save a lot of work. I should also mention that while I'm creating/editing theses files, I keep them stored in Dropbox so I'm in sync no matter what device I have at hand for editing at the time. I go back and forth between my iPad, iMac, and MacBook. Dropbox is a godsend for that.

Clear as mud? ;)