Agree with Notes ...
You could use a separate Note for each next action context as well as one for a Projects list, Someday/Maybe, etc. It's a pretty good little app.
I personally use OmniFocus and while the desktop app has a learning curve the iPhone app (though more expensive than Things) is really straight forward.
Another idea but I'd start with a simple app or one you spend the time to learn so then you can focus better on the GTD methodology (much more important than the software selection).
How did you capture that thought?