I think Metallica and Slayer are what got me into Thrash. I liked the stuff from Destruction, Kreator, Exodus and Vio-Lence. I agree with Vaevictus that bands are constantly trying to evolve, for some bands they follow the "if it ain't broke, don't fix it" mentality. But not all musicians settle for a single style and stick with it throughout their whole careers.
And on a lighter note, I think that Lamb of God are Metalcore, Mastodon are a hybrid of Hardcore and the more extreme metal styles, and Meshuggah are metal with Death and Thrash influences but not full-on Thrash.