Lydia,
Thanks for your kinds words, but please do not mistake me for something that I am not. There are many GTDers, and everyone is different. I am sure there are many whose thinking is similar to mine, but probably even more people who think totally differently. And we all honor GTD. You should be asking the people you would like to be helped by your app. What kinds of users are you really - really - after with GTDsimple?
At first sight GTDsimple strikes me as unusually pretty. That's a big plus, but it is takes more than a pretty face, of course.
It seems that GTDsimple - like quite a few other apps, but I cannot remember all the names - starts with addressing the ultimate super-noob and devotes a lot of attention to pointing out things like "collecting" and "actionable?" and "deferring" etc. I think that is probably great, but I am not totally sure. I am sure there is a market for it. FacileThings.com and Zendone.com are two companies that do precisely that brilliantly and seem to be appreciated for it. And I, too, appreciate people who are thorough in what they are doing. Doit, the app I am using myself, also covers that, and a million other things that I do not really need, but much less coherently and consistently - but overall they have a feature set that covers my needs a notch better than most.
The question for me (and some others, I am sure) is what follows behind that pretty surface. My two most recent apps have been Doit and Nirvana. Had I had a Mac I would probably have tried Things and Omnifocus, too. Those are the kinds of apps I tend to like most (or least little). I like Zendone in many ways, too, but not completely (they focus too much on reference integration etc for my taste). I am looking at GTDnext (quite similar name to yours) mainly because it has action hierarchies, which is something I like, not only for subprojects etc, but also for representing the 20 k and 30 k horizons.
Another thing I like in a GTD app is priority, and Doit has it. I use this as a review frequency indicator - most useful for me; most GTD apps do not dare to have that because David Allen perfectly correctly has pointed out negative things about some other, very specific types of interpretations of the term priority, and most GTD followers therefore shun priority in all of its forms like the plague. (David emphasizes priority especially at higher horizons, and plays it down for situational decision making during the day, and very strongly advises against priority staging, i.e. first A, then B, then C. )
One field I am very interested in a future GTD app is tagging (contextual categorization) and filtering. It seems to me that GTD perfectly correctly avoids soft, subjective planning dates (and, yes, indeed, I have always hated those, too) and instead focuses on situational decision making (context, time, energy, priority). Most apps (all, actually, as far as I know) have extremely crude tagging and filtering capabilities for such situational factors, though, and this leaves many users complaining about the length of their lists. One of the simplest and most powerful features an app could have is elimination filtering (NOT filtering). This would allow you to create a single and accurate list of suitable actions to consider this morning - allowing you to exlude @errands (if it is raining) and @John and @Mary (if they are sick) etc. To see ALL the good choices on ONE single screen. Why doesn't anybody have that? Exclusion filtering would also allow you to tag tasks much more precisely without incurring additional tagging work (e.g. if you tag a small minority that require @Silence you do not therefore need to tag all the rest with @Noisy just top be able to find them - you just exclude @Silence. But apps never have this exclude filtering, only include.)
Lydia, what are your plans for these types of features that would interest the fraction of GTDers that I belong to?