I read GTD and it changed my life, but I'm not a strict adherent. The biggest thing I learnt from GTD is only having the next task for any given project on my to-do list.
I've seen that manual task sorting is on the most-frequently-requested list, but I'd like to put in my vote to have something a little smarter, that is the ability to say that Task Y is dependent on Task X. (Or to use Project Management speak, Task X is a predecessor of Task Y.) The logical outcome of this would be to only show tasks on my hotlist which aren't waiting on a predecessor, i.e. hide tasks I can't do anything about yet.
I've just upgraded to a Pro account today, so obviously I love the product already, but if you add this feature for Pro Plus accounts then you're guaranteed to get me to upgrade again!