Nice work Tom
I think the point of finding a "perfect" GTD app is going to be difficult. We're all different and evolving in our own way and how we manage things. It's difficult for apps and developers to keep up with the human brain ... Unless you get someone who is a savant in this area who is always ahead of the curve (any volunteers?)
Then again, you've done a great job identifying the 10% missing in most cases ... so perhaps my glass is currently half empty.
I can see the effort you put into this an it is remarkable... again, well done.
I was using Any.Do but I make fairly simple lists. I've just update my phone to ICS so trying Astrid again. (I'm on annual leave so I have to time to dabble a little)
But when I go back to work I have to follow David's advice: "Make it up ... Make it Happen!"