I agree with Markdown! I've gotten really comfortable and efficient entering text using Markdown.
I tried copying some of my notes to GTDNext from my previous system and there's some tools missing from the current editor - like headings, horizontal lines, block quotes, and remove all formatting (having to go into HTML view and manually remove tags is cumbersome).
I do like GTDNext's table tool though, that's one thing that's missing or too limited in a lot other apps.
It would be perfect if I could just copy the raw markdown to GTDNext.
There are some interesting Markdown implementations out there, look at Checkvist and Notejoy. And don't forget to use a more complete Markdown parser, like Github's GFM, rather than Gruber's or CMD's basic parser.