Then let me rephrase: I can't believe so many people fail to realize the influence that Metallica had. It's a shame that people let Metallica's mistakes cloud their judgment. Such people have a propensity to fixate on those mistakes and let all of their accomplishments go unnoticed. The fact of the matter is that Metallica's one of the best heavy metal bands ever, and the genre wouldn't be the same had they not rose to prominence.
Slayer's equally influential, but I personally think they're overrated.
Metallica = Very intricate arrangements, extremely well written music, awesome guitar solos with both shred and melody, and often thought provoking lyrics.
Slayer = Extremely aggressive and awesome stuff, but not as good as Metallica. Guitar solos are sub par and the lyrics contain superfluous satanic references. In my opinion, Metallica is superior.
As far as influence goes, however, I think Metallica and Slayer are pretty much equal.