Opions?

[ZiNgA BuRgA]

That's a nice idea RateU.
I think I may have an idea about the URL scheme.  I think, the default filter will only be applied if no filter has been supplied in the URL.

So using the example above, if the default filter is:
filtertf_tag=Cheap
filterxt_prefix=3

And someone visits forumdisplay.php?fid=10&filterxt_icon=2, there'll be one filter active (the default filters will be disabled).

What I meant by variables is whether to allow them in the default filter string, eg, you specify a default filter of filterxt_uid={$mybb->user['uid']} or similar.

For your idea, maybe I'll have a key representing whether other filters are active in the filters_set array.