Would love a Date sort! I also would love to organize hierarchical lists. The organization available in Diigo is very very flat. My top 3 features in order of importance would be:

1. Automatic thumbnailer (not manual screenshot) of all bookmarked sites. Thumbnails then automatically appear in library and lists with the link to the site.

2. Nested Lists

3. Date Sorter

All this is easily do-able from a programming perspective.