Master quality good enough for commercial television, movies or other uses.
Something different and original that other people can't do. They probably get a lot of crap from people with things like GarageBand or Band-in-a-Box, so the samples and synth patches you use should either be brand new, or original. ie, if you come up with an amazing song that gets picked up by a publisher for use on a national commercial, and the advertisers musicoligists determine that every sample you used was included with the latest version of Halion, chances are they're gonna turn it down. Think about what the music is going to be used for and compose for that purpose.. ie, sports video highlights, commercials for deoderant, theme music for a Tv show, backup music for a documentary, mood music for a website... etc etc..
Come up with varying lengths of demo's. 30 seconds, 60 seconds etc..
Learn about stems and supplying alternate versions of your music and on the demo get straight to the music as soon as possible. ie, don't fade a song in with sound effects or mood music before launching into a rock tune or whatever. You can provide an alternate mix on the stems, but the first guy you have to get the music past is the part-timer with a list of 'rules' he has to follow.
I guess last thing is never take anything personally no matter how much of an ass they are when describing your music or the reason for turning it down.
You can always sell it to somebody else and probably for a lot more money.