This came up recently (and I thought Steven asked me to expand on it, but of course now I can't find the source...)
Anyway, here are the big issues I can see as soon as I start messing with the filters again:
1) Changes to layout (set displayed fields, etc) are not retained. No changes to any of the filter settings are retained, either.
2) Filter is easily lost: Any action which triggers a refresh of the main panel resets the filter to show all tracks. If you navigate away from the filter, you can't get back to it except by going through the Tools menu again. (In other words, you can't really do anything else while you're using the filter, or it gets messed up.)
3) This is especially problematic with the "small artwork" filter, since it scans the entire library EVERY time you load it.
I would love to see these filters improved, because they could be super useful. Right now they're mainly useful if you select all and perform a single action on all the tracks.