I think planning is the key! A party that is organised will run smoothly. If children are occupied, having fun and being looked after properly, then mums and dads can relax and enjoy some adult company and conversation. As a busy parent that is what I think makes a great party.
For the kids some ideas might be to set up a movie room, snack table, computer games and activities, jumpy castle, maybe pony rides, face painting, balloon animals, clown or fairy and some music.
For the adults maybe music, board games, adult snack table, drinks, plenty of nice comfy chairs for some chats and laughs.