I think it is a big deal that it is possible to search for collections, and that there is a collection filter in the advanced search.

However, presently when setting up the collection filter in the advanced search, collection list is presented as a flat list. This presentation makes it really hard to find desired collections. At the very least, prepend each collection with "-" * 4-8 * collection level, such as:

Collection 1
---- Subcollection 1 Level 1
---- Subcollection 2 Level 1
-------- Subcollection 1 Level 2
---- Subcollection 3 Level 1
Collection 2
---- Subcollection 1 Level 1
-------- Subcollection 1 Level 2

Or, perhaps, even something like (using monospaced font)

Collection 1
|---- Subcollection 1 Level 1
|---- Subcollection 2 Level 1
~~~~~|---- Subcollection 1 Level 2
|---- Subcollection 3 Level 1
Collection 2
|---- Subcollection 1 Level 1
~~~~~|---- Subcollection 1 Level 2

where "~" denotes the space character.

For example, such a formatted list can be generated with pure SQL. It can be saved as a view, or executed by the calling code.