Its more in the timing, the rhythmic value, the groove, than it is the notes or chords. Most note or chord pattern combinations have been done, its in the rhythmic feel behind them that can be more origional and less cliche. This is one problem early on in your playing, later as you become more rhythmically advanced and groove oriented you might come up with something catchy. Many professional metal bands and other styles of music have cliche, rehashed riffs with typical timing, from there its the vocal application and little things in the arrangment that could still make a decent song.
I have written two decent songs in my head away from the guitar that I then spent the time to figure out on guitar. This was a satisifying experience and they really did come out well. These came from vocal and melody ideas and all came together at the same time, giving me a completed song. I cant seem to get back to where ever I was in my head at that time.
Most of my other stuff has been that which for what ever reason just comes out from open jams. This has been rewarding too when at times they have been a long, well flowing nearly complete guitar rhythm piece, right out of the clear blue within 10 minutes. But then stalls out for months with no lyric or vocal direction which is not my strong suit to begin with.
I have also nearly always been blessed with a few good friends that have been drummers I played regular with. My best friend was a excellent natural drummer when we were teenagers so I had someone to rhythmically guide me and feed me from day one. I can't say enough about the benefit of being able to interplay with a drummer to get a groove on and be creative. It a form of inspiration I guess. I have used them like cheap whores....... lol now some of my stuff stumps drummers until they get use to the timing and feel changes