The filter bar at the top is wonderful. It allows you to quickly filter tasks within the list you are looking at. You can use this filter both for ad hoc purposes and for simple and frequent routine purposes. It saves you the effort and menu clutter to define and save every little possible filter as an advanced filter. Very good.

But currently you can only use it as a "pick these only" filter (Boolean AND). It could all be much more versatile, and much more useful, if Boolean NOT and OR were also possible (for example using alt-click and shift-click for these):

The most important addition, in my opinion, would be the Boolean NOT, i.e. be able to use the filter bar to eliminate tasks with certain tags, for example to hide errands if it is raining and you have decided not to go out, and then hide phone if your phone is broken, hide John if he is sick etc.

Indirectly, the capability to eliminate things from the list would enable you to devise a lot of useful little tags, and still not have to tag more than a small minority of your tasks with these tags, only tag the exceptional ones. For example, if a minority of your tasks require extreme quiet, then you could tag those few tasks with the special tag Quiet. If you are in a noisy or normal environment you could exclude these by filtering for "NOT Quiet" (alt-click the Quiet tag). If, on the other hand, you happen to have a very quiet moment and would like to make the best use of it, you would filter for Quiet just as we do today.

By gradually eliminating tags, one by one, that are inappropriate to consider at this very moment, it would be much easier to pick suitable tasks from the Next list.

Boolean OR would also be useful. If you happen to be meeting with John and Alice, and want to see all tasks you need to talk with either of them about, you could simply filter for John (normal click) and then "OR Alice" (shift-click), which would show you all tasks that have one or more of these tags.