* Free (beer/speech)
* Self-hostable, including the sync server, which doubles as a web-accessible version of the app (++mobile access!)
* Extensible (want UI elements that do things with your notes? Easy. Want to render graphs based on metadata you've entered? No problem. Your own metadata and tagging scheme? All built in.)
* Encryptable (built in encrypted notes function)
* Syntax highlighting for most languages you probably care about
* Exportable, no proprietary formats
* Notes can be written in markdown
* Truly cross-platform (Mac/Linux/Windows/Web)
The tiniest of gripes can be directed at things like requiring you to use a key combo to insert a link (rather than letting me, say, use some markdown and autocompleting it), but that's minor. This blows the Evernotes and the NValts and quite possibly even OrgMode out of the water.