I've been playing for about a month and think that custom search would be extremely useful. As said before, AR and CS etc., but also by numbers of circles / spinners / sliders (for example you could want a medium length song, and search for circles between 300-700, with everything else on any). Personally I prefer maps with low amounts of spinners and would like to search for them without going by hand.
Also, a search to limit your songs by type (osu!, taiko, etc.) would be good.
Also, a search to limit your songs by type (osu!, taiko, etc.) would be good.