Guitar. The tuning of the low E string down to a D. This allows one to simply bar one finger across the 3 low strings (D, A, D) and create a simple and powerful chord, 1-5-1.
Drop D is generally used by talentless nu-metal
bands who cannot actually play anything noteworthy.
System of a Down uses a lot of Drop D. Maybe that's why they suck so bad?
Alternate tuning isn't always bad. Real bands like Led Zeppelin use open G and open F for slide guitar and harmonics with advanced picking.