What bands can be considered gateway bands, bands that first bridge the gap between shit and metal? I think Slipknot and Korn are obvious, as well as Pantera. Also, Led Zeppelin and Black Sabbath/Ozzy come to mind.
Why does the gateway to metal have to be through shitty music? Do we have that low of an opinion of the basic concepts of metal that all it takes is for someone to hear some 70s heavy riffs and all of a sudden they are ready for metal?
How about good gateway music: heavy instrumental or prog rock, experimental ambient and experimental, drone?
Lustmord, Mogwai, Fennesz, Yes, Isis, Earth, Porcupine Tree, Rhys Chatham anyone? All could be considered gateways to metal, and all extremely good. They actually make music that has value, unlike most of the rest of the stuff listed in this thread.
Looking at those bands is probably enough for anyone to understand how I listen to almost exclusively black metal, but maybe I am expecting too much from this crowd.
The Used are an emo band (don't start Mort), who got me accustomed to harsh vocals because they have screaming. I still have a soft spot for them although I wouldn't buy any more from them.
They're a pop rock band with an excellent debut and shitty everything else.