I've built Wren (https://github.com/bjesus/wren) with a pretty similar idea of simplicity in mind — a task is just a file — but, it can also be whatever kind of file you want:
1. Drag an email to ~/Notes and it's a task
2. touch ~/Notes/get\ milk and it's a task
3. ln -s ~/Documents/something-i-need-to-finish-writing.doc ~/Notes/complete\ writing and it's task
Yeah - I mean I haven't been adding any new features recently, but mostly because the system just works. I'm using it daily and definitely fixing stuff if they break.
1. Drag an email to ~/Notes and it's a task
2. touch ~/Notes/get\ milk and it's a task
3. ln -s ~/Documents/something-i-need-to-finish-writing.doc ~/Notes/complete\ writing and it's task
Wren doesn't care about the format.