It looks like a great tool. I was actually looking for something like this the other day, and disappointed with what I found.

I write for a living, and I've vacillated between a few different distraction-free writing tools.

Writeroom http://www.hogbaysoftware.com/products/writeroom - was the first distraction-free writing tool I encountered, and, not coincidentally, the first mac app I actually purchased. It's a very good piece of software, though it takes a bit of work to get it configured into what I find to be a visually appealing workspace, and now that I've experienced iA Writer's markdown support, I'll probably never go back to Writeroom.

iA Writer http://www.iawriter.com/ is what I've been using for the last month or two. I love the way it looks straight out of the box, and adore its support of Markdown, and its focus mode (wherein the paragraph you're writing is always vertically centered, and everything else is greyed out). The only features I feel it lacks is the option to export to Rich Text, and remembering my preference for checking spelling as I type. (Also, I feel that its pricing is a bit out-of-whack. While $17 is fine for someone like me who writes for a living, I'll bet they'd more than quadruple their sales if they dropped the price to $4.99) EDIT: They've dropped the price to $9.99, and added RTF export in a Sept 1st update. Clever.

QuietWrite http://www.quietwrite.com/ is the most direct online competitor to LightWrite. It adds some useful features like maintaining a document library with your login and whatnot. Sadly, its habit of bringing up its context menu every time I move the mouse bugs the hell out of me, and I'm not a big fan of its green'ish color palette.

To become the online app that replaces Writeroom and iA Writer for me, I'd like to see:

* User selectable themes. I'm not a designer (a trait I suspect I share with most of your users), but I have visual preferences, and if I'm going to stare at your app for a few hours a day, I want it to look good. Since "look good" is a subjective thing, I think a few carefully-designed theme options for fonts and colors is the way to go.

* A login that enables a document library, with sorting. Your power users, aka, people who would be willing to pay for your app, will need to keep their documents organized. Folders and/or tags would do the trick.

* Easy offline saving. I backup all my local documents, and I'd feel much more secure knowing I had a copy of my LightWrite docs saved locally. Make it automagically synch with my DropBox, and I'll be in love.

* Markdown Support. I know that in some ways, any type of formatting goes against the distraction-free writing aesthetic, but I find it essential for any serious writing tool. Whether you're writing technical documentation, fiction, advertising copy or personal correspondance, basic formatting tools help keep your document organized. Markdown lets you implement intra-document organization without any context menus cluttering your desktop. Adding Markdown support for citations would - I suspect - make your app a great option for the academic subset of your potential users.

* Export to HTML, TXT and RTF. In my line of work, the copy I write is going to a designer, a blog post or a client. These formats would cover my bases nicely, and I suspect the bases of most of your other users.

Good work, and good luck.