The example at top could also be said this way:
I can tolerate the heat, but I can't stand this humidity!
Using "what I can't stand is ___" places more emphasis on the thing that the speaker doesn't like. It makes the sentence sound like a more forceful complaint. Here are some more examples:
I'm OK with working late hours from time to time. What I don't like is my boss expecting me to do that with no overtime pay and no appreciation for my effort.
I don't want to hear your excuses. What I want from you is an apology.