Well, metal is a staggeringly diverse genre, and you're comparing doom/proto-doom to thrash metal. Compare Black Sabbath's
eponymous song to some
modern doom metal, and methinks you'll see a stronger resemblance.
Yeah, compare Sabbath to a band like Sleep and a thrash band will look out of place. But at the same time take something like the outtros to various Sabbath songs like Iron Main or Heaven and Hell (late I know) and you hear a prototypical high-tempo speed/ New Wave of British Heavy Metal sound (tell me the bassline of the Iron Man outtro doesn't remind you of Maiden). There is absolutely no reason why Black Sabbath shouldn't be considered at least a progenitor or perhaps one of the first "true" heavy metal acts.
The way I look at it is you have three methods to heaviness as far as beat pushing is concerned:
1. There's the dragging style of doom which sort of pulls you down with its slow tempo and behind the beat lagging
2. There's the static thunder of bands like Meshuggah which are metronome perfect and kick you right in the chest,
3. And then the ahead-of-the-beat, driving speed of thrash which makes your heart race.
No band which employs one method will feel remotely similar to one which employs another. Even Meshuggah's early music, featuring thrash orchestration feel decidedly different from a Metalica/slayer style band. At the same time their later mid tempo and slow music doesn't drag you the way a doom band will. No matter the tempo or orchestration, Meshuggah always feels like a piledriving machine.
As a comparison. I pretty much consider Meshuggah the standard as far as straight tempo heaviness is concerned
Slayer vs. Thrashy Meshuggah. Note the total tightness of Meshuggah, vs the loose chaotic thrash of Slayer
http://www.youtube.com/watch?v=K6_zsJ8KPP0
http://www.youtube.com/watch?v=vOd-T58qHLA
Sleep vs Downtempo Meshuggah. This is a less valid comparison musically, just note the tempo feels (and the sabbath influence)
http://www.youtube.com/watch?v=lklU89L8sXs&feature=related
http://www.youtube.com/watch?v=lklU89L8sXs&feature=related
The way metal bands approach the beat is integral to their sound, and they are three very very different kinds of heaviness. None is less valid than another.