As a (talentless) guitar player myself, I've always been more fascinated with powerful riffs than sweep picking, tapping and other tricks of the virtuoso trade. That's not to say I scoff at them by any means, because they can be quite effective when implemented where I see fit. But when a guitarist can cut a heavy, bruising groove with much less technical effort, I find it much more satisfying. A great example would be Matt Pike from Sleep and High on Fire. That guy can make it sound like eternity is crumbling around him with little more than a Les Paul and a Green MatAmp.
And I agree, Sabbath are the fathers of all metal as we know it today, though I never really got into the Dio era of the band nearly as much. It was good and certainly valid, but it didn't do as much for me as the original lineup's direction.
As for "screaming", I take it purely on a case-specific basis. I guess the main thing for me is that it has to sound like the natural sound of the band, it can't sound like they're trying to sell me on how brutal they are, like they all figured out their parts and then the vocalist just said "yeah, and then I'll scream as hard as possible". And I hate those chug-a-lug bands that just go "dun-dun... dun-dun... dunnnnnn" while the singer just screams nonsense, and then they immediately go into the 'sensitive guy' chorus with arpeggios and the singer starts whining like the asswipe from Fall Out Boy. A great example of a band that gets it right IMO is Skeletonwitch. The guitars, bass and druns are a super tight and clear, but ridiculously heavy at the same time. It feels natural that Chance Garnette would be be screaming demonic lines about being "born of the light that does not shine" over the top of it, and it works because he doesn't force it. You can actually understand him most of the time, and he doesn't overdo it. It all makes for a perfect blackened-thrash sound that just doesn't get old, even if the formula doesn't change much from record to record.