Wow, there are a lot of misconceptions in this thread.
Tuning low doesn't necessarily make the guitar sound like cr@p. If you're using inappropriate strings or bad equipment or just don't know how to use the equipment properly, then yeah, but it's entirely possible to get a heavily detuned guitar to sound good. I tune to drop Ab and Eb below that (yes, an octave and a half step below standard), and I don't have any problems with it. Meshuggah's tone is massive. Stephen Carpenter tunes lower on every album, and his tone is as clear as it is huge.
Tuning lower isn't necessarily compensation for lack of skill. Yeah, there are far too many kids who just drop their guitars down to C or so and start threading lame breakdowns together. Don't make the mistake of thinking everyone who detunes is like that. Also, if you set up the guitar properly and use the right strings, playing on a detuned guitar shouldn't be any easier or harder than playing on one in standard. On the other side of that, standard tuning doesn't necessarily mean that you're going to be playing '80s metal. Yeah, they tend to stay pretty close to standard, but they're not the only ones. Not only do I like tuning low, but I like playing in standard, too. Hell, a lot of the time I play with a capo on a standard tuned guitar. I don't play hair metal.
To reiterate some stuff that has already been said, detuning just offers a different kind of sound. I couldn't imagine some genres sounding right in standard tuning. It's entirely possible to write good and heavy music in standard, and any good musician should be able to, but there's nothing wrong with detuning to achieve a different sound. To borrow from another post, it's like saying that a piano with more than X number of octaves of range is stupid. You can find plenty of examples of whatever generalizations you may want to make, and I'm fairly likely to also dislike those examples, but seeing examples of people who detune for the wrong reasons and can't do it right and assuming that the same is true of everyone who tunes below D is silly.