have tried many of these apps, Things, Wunderlist, Omnifocus, etc etc... at the end 2Do is the one I'm feeling most comfortable with - an intuitive interface and a good balance between features and clutter, and sync with iCloud so I can use Siri (and I feel this was the deciding point).
Some easy things that are still missing:
1) you should be able to insert links to emails and contacts the same way you can do for files or URLs
2) the quick task feature should capture the frontmost URL, email, file, etc. automatically, the same way Things does.
3) you should be able to snooze/reschedule notifications easily, be it in the notification itself or in the 2Do window
4) when you click on a file url it should open the file, not just show it in the finder
and the not-so-easy ones of course:
1) teamwork: contacts, delegation
3) task sequence templates: i.e. you have a task template called "send quote", when you mark it as done immediately another one pops called "follow up call", and so on. Or at least some option like: if you press and hold the "done" checkbox, a pop-up appears to create a follow-up task (marks this one as done and creates a new one for 1d, 2d, 3d, 1w, or so depending on your selection).