If I remember correctly, in Karazhan the only boss you had to do to progress was curator all the other encounters could be skipped and done in any order you wished.
Moroes, Curator, Chess Event and I believe Netherspite were not optional. Aran opened up the shortcut to the top..ish. Probably more non optionals, but killing the bosses also eliminated the trash linked to them for return trips.
Moroes, Opera, Curator and Chess were not optional if you wanted to get to Prince.
Aran opened up the teleporter (once they added it).