Desired Feature: Elimination Filters AND/OR/NOT
Most GTD apps and other task manager apps have some form of "filtering" that allows you do narrow down the selection of items that you see on a list. Usually, these filters are implemented as "pick this" filters, for example show only those tasks that require Low Energy, or show only those tasks that require John, or only those that require you to be "out" (errands) or at home. Etc. And that's good. Very useful. For example, it allows you to see only errands if you are going out and need to see more things that you might also consider doing while you are out. Or it allows you to see what other things you might want to talk to John about now that you happened to bump into him.
In many apps you can continue filtering by ctrl-clicking yet another tag, which typically results in a Boolean AND condition (i.e pick only those that also have this other tag), or a Boolean OR (i.e. also pick those that have this tag).
But this is not the only kind of filtering that would be useful.
In general, in GTD, you finally pick tasks to do now by looking at the four factors Context, Energy, Time and Priority and weighing together these task requirements against the current situation using your gut or common sense. (And obviously you also look at already committed actions such as appointments or agreed time blocks such as working hours.)
The easiest way I can think of to narrow down the list to a reasonable number of truly relevant tasks is by a process of elimination. In technical terms, this could be implemented as the ubiquitous "pick this" filter, thereafter allowing you to continue filtering using Boolean AND/OR/NOT, for example using ctrl-click, shift-click and alt-click respectively.
Process of Elimination
A process of elimination is generally very powerful. In a way, GTD already employs this principle when it comes to creating the Next list. All non-Next things are "eliminated" during processing (done by others goes to Waiting; not sure goes to Someday/Maybe; Not yet goes to either Tickler or to the "subsequent portion" of a Project). This leaves you with a Next list which only contains those things that you are willing and able to consider doing right now.
But not quite. The Next list still usually contains items that are temporarily inappropriate to consider, for example low priority errands if it is raining and you do not want to get wet, or things to discuss with Alice who however is having client appointments the rest of the afternoon, or things that require much more mental focus than you can muster right now. Etc.
It would be natural to continue in the same vein and eliminate the inappropriate tasks. It is often easier to eliminate the obviously inappropriate than to immediately pick the most appropriate.
Unfortunately, with only the "pick this" type of filter you have to use "random fishing" (or have an immense and unmanageable number of overlapping tags). For example, if one task requires you to use the phone and another task requires you to use the internet, and a third one only requires you to be anywhere in the office and another task is something you need to do together with John, you would often be quite willing and capable to do any of these, but perhaps not be prepared to do tasks that require you to be abroad or at the golf course, or things for which you need Alice, who is out. You could use "random fishing" and look first at tasks that require the phone, then at tasks that require the internet etc, but that requires you to have a very good memory since you cannot see them all at once.
Elimination - the NOT filter is a particularly important here - allows you to keep the things you might want to consider on one single list and hide the rest.