Be able to hide what communities you are in so people can be in communities they wouldn't want to make public. Especially support communities where people might not want to reveal to the world that they have certain problems.
This was brought up in http://www.livejournal.com/talkread.bml?itemid=6811081 but Halkeye said he needed it in template form. Everyone likes it. Please get official go ahead and just do it.
People all over LiveJournal rejoice and peace claims the land.
Accolades will pour onto the coder who does this.
And people will feel more comfortable joining communities that they do not want to be publicaly listed as in.
Less honesty and openness in the world
People won't realize that 90% of the world is just as messed up as they are and that it's okay to have problems.
Someone has to do it.
Details for implementation:
Approve it and then bribe developers with cookies until it is coded. Or with comments. I'll even give halkeye comments that are relevant to the journal entries.