Cover grid, virtual libraries, customizable details pane (select the fields to show), filtering (both tree and search), virtual libraries (collection), export template, metadata editing and embedding (write to file).

Thats the few stuff I used regularly.

Not the OP, but my library management is 90% tagging, format conversion, metadata fetch and editing, and format tune-up (font subsetting or stripping fonts, fixing covers, etc.). Virtual libraries keep me organized based on tags.

Hey, which program do you do that now with, Calibre or a set of separate tools?

Calibre.