I would like to suggest enhancement to the way Autofilter works. Let's say I 
want to filter by two columns simultaneosly, first by one and then by the 
other.
I think that showing in second filter only values, that are visible after 
application of the first one, is much more logical (what is the point of 
showing empty subset?) and would allow user to focus only on relevant data.

On linked sample file click arrow down in cell B2 and note how "b1" and "b2" 
values are available for selection. 

Thanks a lot.
WBR,
K. Palagin.