I'm with Core2TOM on this one.
We should be able to search by the same criteria as is available in the editor.
Having more difficulties to choose from will be better so we can more precisely
choose the difficulty we want to play.
If you make it so a player can not upload tracks of a given difficulty,
unless they first beat X tracks from career of that difficulty*,
that will make it so players are at least familiar with
how to properly label their tracks.
As well as filter out many of the trolls who don't stick around long
enough to know how to make (or beat) a good hard/extreme/ninja track
from publishing them.
*


