What it used to be:
Talking about the weather used to be the paradigm of meaningless, idle chatter. Someone would say, "Boy, it sure got cold fast!" and you would reply, "Sure did! But you know, I really don't mind the cold," and that would be the end of it. There was no need for heated debate. Everyone went around giving their two cents, not even really listening to one another, just waiting for their turn to talk; it was like operating a conversation in "safe mode."
"Another lovely day in paradise."
The weather was the perfect topic for small talk because we were all in the same boat. Everyone had their opinions, but the beautiful part was that our opinions didn't matter. Kind of like whether or not it's OK for you to bone in your backyard: Your neighbors could have all the opinions they wanted, but it wasn't going to change what went down. So everyone got along great.
What it is now:
Your college-age cousin yelling "We're destroying the planet with carbon emissions!" while your mom calls him a hippie.
The second someone mentions the weather, run. The conversation will start out harmless enough, but because of the debate over climate change, you've got about two minutes to go from "Some weather we've been having, eh?" to "Exxon is weather-Hitler!"