I'd just like to chime in and say that Celtic Frost-- one of the heaviest bands ever, imo-- tune to E standard for the first three classic records and sounded TOTALLY huge. Of course, a big part of that was Tom Warrior rolling his tone knob completely off, but I digress...dudes never detuned until Monotheist in 2006.
Heavy sound is in the rythm first, the tone second, and the tuning maybe third. The reason it is common now, espescially drop what have you, is to simplify the guitar players job as far as I am concerned. I recall reading an interview with Sully from Godsmack who said he couldn't play at all until some guy showed him this "cool way to tune the e string" where he only needed one finger for power chords (kinda puts their music into perspective). Having a good "kick some guys ass" breakdown is the key to a heavy sound, generally followed by a searing fret wanking lead rythm at 200+ bpm. DOn't get me wrong, I love metal of all shapes and sizes, but when stuff written in standard E and played through an amp with a little overdrive gets the crowd moving justas much as drop mud and 15 different distortion effects at once, it would appear that tuning is almost moot.