Medadeth. I love Slayer and Metallica and I have tried really, really hard to get into Megadeth. I don't know if perhaps I am not getting their best albums, but everything I have heard so far from megadeth is good, but not great. Also, with metallica and slayer, i get their point, i see what they are doing with their music. I can't see that with Megadeth.
Another band is Marillion. I have tried really hard, but everything seems "too calm" for me. I recognize their importance for progressive rock and it is a shame I can't understand their music, because I would certainly love to.
Neurosis. I love Tool, Isis and Rosetta... I just don't get Neurosis.