My previous limits were:
1) no lower than 30 degrees
2) nothing frozen on the ground
3) no rain if it's less than 50 degrees

This year I got some Conti Nordic Spikes on my commuter-tourer road bike and my rideable zone is wider.

I've done down to -2 on my commute in, which had me thinking possible frostbite, so at this point I am deterred by temps less than 20 degrees. Some new gear could bring that down into the teens.

The Contis are good on packed snow and ice, but not so great in deep snow.
My Plus bike ("almost fat" - 3" tires) is good for deeper/fluffier stuff but not good for packed snow or ice.
So I can do hard/slippery or deep/fluffy, but not both.
Typically my commute gets cleared, so usually don't have to deal with deep/fluffy.

Where do you draw your lines?
How have your lines changed... and what was the gear or attitude change that helped you do that?