Communication for sure. I think one of the hardest skills to develop is being appropriately critical without coming off like a jerk. This is especially true in remote teams where even the best conferencing solutions can't transmit subtle cues like body language.

I have been going about this by devoting a lot of time (maybe triple what I would intuitively expect) to writing thoughtful comments on code reviews/issues. And, while doing so, thinking through how I would go about the same conversation if it were in person.