Pro 3.3
Scale-based symbology is a really great tool.
Being able to change the symbols in each layer depending on how far zoomed in you are is awesome and saves a lot of map clutter.

It has, unfortunately, two major drawbacks.
The first is that it isn't available when you symbolize by single symbol. That'd be fine, because you could get around that, if it weren't for the second issue:
Scale-based symbology is unavailable for the "All other values" symbol class.

Please add it in; I want to have the same flexibility for my "everything else" category as I do for the "stuff I care about" categories.