I remember when 6th edition was coming ito its cups that some tournaments and organized event attempted to curb overly enthusiastic power-gamers by allowing other players to evaluate their opponent's army, effectively assigning it a "composition score".
The idea is good: if you want a good comp score you take a non-abusive list and instead build it around a clear theme (as if those two were completely distinct and opposite. Sigh). It doesn't take a genius to figure out that anyone with a "win-at-all-costs" mentality will consider a low comp score a reasonable "cost".
I've noticed a change towards trying to nip abusive lists in the bud, as it were, by introducing "composition restrictions" where there are limits on how you can build your army. I don't much approve of this approach: to me comp restrictions incite me to find the most abusive combinations allowed rather than making armies I'd like to play.
Aside from those proactive and reactive comp mechanisms, there seems to have emerged a third meaning of the term: That "comp" is somehow the same thing as nerfing the game (I think we all know what "nerf" means. If you are reading this you are surely aware of the existence of the Internet).
So, when you say "comp", what exactly do you mean?