I haven't been able to read all of the previous comments, so please forgive me if these are duplicates.
I had this problem yesterday and lost a world:  I clicked the Version button on Monster to see what the latest number was, and then put it back to 1.0.5 where I had some extra mods installed, and hit Launch.  The launcher immediately started downloading a new version of the modpack, and when it was done all of the extra mods I had added were gone.  It would have been nice to be asked when I hit Launch whether I really wanted to download the pack again.  I'm not sure what happened.  Now I've lost a world that I don't really know how to get back.
I've gone into a few of the private packs, and now they're just cluttering up the list.  I wish there was a way to delete "instances" from the pack list.  I'm sure if I went into some config file I could figure out how to delete them, but it would be much easier to click one of the packs and hit delete.
Also, in parallel to the last idea, I wish there was a way to create a copy of an instance and disable automatic updates.  That way I could add mods to A pack, but not THE pack.  I've tried MultiMC to create a copy of an instance, but none of the mods load so I obviously don't know what I'm doing there... 
 
Love the game, love the launcher, wish I had more understanding of the configurations so I wouldn't lose worlds like that...