I understand too cold. I've taken creative steps like a plastic rain poncho under my jacket and latex gloves under my riding gloves to lessen the wind chill and keep my previously frost-bitten fingers from freezing.
I understand too wet when it's raining buckets and visibility is poor and hydroplaning is a serious safety issue.
Yes, dehydration is a concern in the heat. Drink lots of (non-alcoholic) beverages. Wear a light, long sleeve shirt to keep from getting sunburned ... even put it on wet for a bit of extra cooling (jeans too). Wear a half-helmet and good riding glasses instead of a full face and let the wind flow through.
Too hot to ride? ... Only if my air cooled bike overheats!