Probably the most prominent feature that's immediately obvious is the widescreen view. Notational Velocity places a list of notes in a column above the text editor, which doesn't map very well to monitors that are a lot wider than they are tall. nvALT has a side-by-side view that probably makes a lot more sense for most users:
Frankly, for me, the widescreen view alone was enough reason to switch to nvALT, but that's not all it has to offer. There are a number of other UI tweaks and customizations, such as a keyboard shortcut to show/hide the note list and some minor bug fixes, but the big news is support for users who want to write in Markdown syntax.
Markdown is a plain-text markup language invented by John Gruber, the "needs-no-introduction" author of Daring Fireball. In a manner similar to that of MediaWiki, Textile or BBCode, it distills the complex markup of HTML down to a simplified, readable subset that can be automatically converted into HTML format for posting to the Web. For example, the HTML sequence <em>emphasized text</em> is written as *emphasized text* in Markdown, which is both easier to read and easier to type. Markdown has gained a lot of mindshare in Mac circles over the years (see previous coverage on TUAW) and nvALT brings robust support for it to Notational Velocity.
nvALT can take a text file containing Markdown sequences and produce a formatted live preview window, with customizable HTML and CSS. It can then expose the raw HTML of that preview so that you can copy and paste it to blogs that support direct HTML posting. Other blogs like Tumblr or WordPress either support Markdown natively or have plug-ins to do so. If you prefer, nvALT will support the alternate markup set Textile, or the extended MultiMarkdown one.
All of these features combine to make on-the-go blogging and hypertext note creation on the iPad a good deal neater with nvALT in the picture. You can seamlessly move drafts back and forth between iOS and OS X whilst working on them, then when you are ready to post, you can check the preview window to ensure the formatting is fine before copying-and-pasting it into your blog site, emailing the meeting minutes you wrote and so forth. Going forward, Brett has big plans to develop nvALT further in order to incorporate more shortcuts for Markdown editing and the ability to send posts directly to compatible blogging platforms.
Footnote: If you want to use nvALT for note synchronization with your iOS devices, you need to choose one of the two popular ways to sync; one is a folder in Dropbox full of plain-text files (the more common approach), and the other is the Simplenote web service, which comes from (but is not tied to) the app of the same name. With the latter approach, you can use either a desktop client or the Simplenote website; with the former, you can use any text editor on your system. Notational Velocity supports either approach, either by entering your Simplenote account details in the Preferences pane or by telling it to use plain text files and a Dropbox folder for storage, as shown below in my configuration (which is syncing with the Elements text editor for iPad):
Unfortunately, almost all of the Dropbox-powered text editors for iPad have a hard-coded folder name at the moment ("Elements" in the case of my example above). This means that the only way to do this is to fire the app up, let it create the folder in your Dropbox account, move all your notes under it and configure nvALT to point at the appropriate folder. We at TUAW hope to see this become a configurable variable in more apps in the future, if only so that we can easily move from one app to another.