Bippa, also, things are broken if the fall under this:
Sweep: Something that can sweep the pretty much 95% of the metagame, would be overcentralizing, and force you to run silly Pokemon on your teams.
Wall: Something that can wall all of the metagame, would be overcentralizing, and was pretty much good game before the match even started.
Support: Something is supportful to the degree where certain Pokémon would become overcentralized.
Weather is broken because:
A) it lets Pokemon like Manaphy, Garchomp, Excadrill, and soon Keldeo sweep entire teams while under the effects of weather. Without weather this won't happen.
B) Under Drizzle, you have things like Ferrothorn and Scizor living Fire Blasts. Not a good point, but the walling rule isn't what makes weather broken.
C) Weather is supportive to the point where it lets Pokemon sweep entire teams. This ties in with point A.