It would be very useful if the smart collections could work recursively. What I mean is that the filters on the upper lever would work on all collections under that collection and the lower level collections could have additional filters
e.g.
| Landscape
| |- 5 stars
| |- marked red
Portraits
| |-John
| | |-5 stars
| | |-B&W
| |-Mary
|-5 stars
|-B&W
and so forth. This would make lot of organizing much more straight forward.