I personally believe that animes are low rated because:
a) The ones we were introduced with (mainly "Outlaw Star" and "Big O") were great till the end, which made no sense thus giving an impression that all animes are like that. Not to mention one of the first anime movies we were introduced to was "Akira" (maybe one of the anime fanatics can correct me or confirm this).
They have a continuous plot line (unlike "Family Guy", "Robot Chicken", "Harvey Birdman: Attorney at Law", "Space Ghost: Coast to Coast", ect.) which makes it hard to continue if you miss an episode.
b.2) The plot line is long. DB and DBZ come straight to mind when I think of long, drawn out stories ("What will he do for the next 6 episodes?!? Charge up his attack and monologue, of course!")
c) Since they are aired so late at night, there is not much incentive to watch it if you had missed a previous episode.
I understand I'm more focused on the AS slot mainly because that is were Toonami was born. I think more current Toonami took on the less adult animes which is fine, the focus group is kids to teens.