Alright, let me try my best to define "heavy metal" myself.
Let's make this a
Tad easier for me by breaking it
Down into two things which I will define:
heavy and
metal. Since that IS the name of the genre,
After All, it's only logical that we define the genre by those two words.
Heavy: Heaviness is both something in an emotional sense and a musical sense. Heaviness in an emotional sense is a strong, powerful feeling, which is more often than not associated with darker topics and imageries, such as the occult, death,
Destruction,
Sadness, and things of the like. However, in more traditional and melodic forms of metal (
Power Metal, prog metal, etc) The "strong, powerful feeling" can have more of a positive edge to it.
One would only assume that you should match the imageries and themes you want to portray in your music with the music itself; as such, "heaviness" in a musical sense is characterized by a resonant, vibrant guitar tone, which can be achieved through a variety of methods. Sometimes the tone can be overshadowed by the other aspects of the music; keyboards, vocals, even drums, but as long as the deep, resonant sounding guitar is still present and is the backbone of the music, it can still be considered "heavy"
Metal:
Metal itself is pretty much impossible to define in objective terms, but I can pretty much sum up what defines "metal" in two words:
THE RIFFS.
There is a certain riffing style
Possessed in metal that is impossible to describe in words but easy to hear (if your ears are well acquainted with the genre, of course). It's kind of like porn: hard to define but easy to spot. The riffs in metal create a certain atmosphere, one that is amorphous in nature; it can be dark,
Aggressive,
Abrasive, uplifting, entrancing, you name it.
One of the major components that makes these riffs metal is (and I know this will sound silly) the attitude.
This attitude is found in the
Heart of the depressive
Black Metal artist who's only sold 10 copies of his album, the big-name
Death Metal artist, the local retro-thrash group or the
Heavy Metal legend. And although this attitude has no real physical manifestation; there is no denying it exists in all of our hearts the minute we fired up that first Maiden disc or stole that Sabbath CD from our older brothers' collection when he was out.
The spirit of metal is what gives us the want to compose these riffs, and in turn, it is what defines it.
If you are a false, don't entry, or you'll be burned and died! 
