I use a combination of Google's priority inbox and the 'mark as unread' feature. Anything marked as unread is a TODO.
Woeful, I know - and it really doesn't work, but it's stuck..!
That's what I do as well.. in combination of Wunderlist.
If anyone from Wunderlist is reading this - I would love to get a gmail integration + a little icon at the top bar like Fantastical.