I think music is described as "heavy" a lot and it's a pretty loose term. What do you think makes something heavy?
think dethklok when the producer's eyes blow up and all his skin is getting blown around...
Either brutalness, being really extreme, fast and brutal, or having a strong beat that you have to headbang to.
Youll never find the answer, TS. Its all relative to your tastes and what you listen to.

Normal people will tell you bands like Metallica and Megadeth are heavy.

People who like pop music will tell you Muse and Disturbed are heavy.

People who like classical music will tell you that baroque music sounds heavier than the older stuff.

People who like extreme metal will tell you that either fast brutal guitars and drums with tons of distortion will be heavy, or they could tell you that slow brutal guitars and drums will be heavy.

Its all taste, im afraid. What is heavy to you, and WHY?
think of a fat hairy 500 lbs mammoth girl with braces playing death metal and making eyes at you....thats heavy....

in all seriousness, its all in how u perceive heavy...i know some guy that told me his band was heavy....they're watered down emo christian power pop...its embarrasing...