I find that it isn't just the act of communication or writing that's important -- it's the person's ability to step outside their own bubble of code, jargon, details, and synthesize (whether up or down) to give someone else the essence of what they need to know about an issue or problem in that communication or writing. To be able to offer some decision, plan some work, take some conclusion from what has been done so far.

I encounter too many developers who simply start talking about the detailed issue at hand as if I've been working on it sitting aside them for the last week. Well, if that were the case, I would not need to talk to you about it.

When talking to someone not directly involved in your work, the point is that some level of abstraction is needed, and probably some decision is the reason for talking. People need synthesis and summary of the issues at hand, and not recitation at the details level.

This takes extra work and preparation, or at least mental organization -- to summarize for someone else what they need to take away from your details. It takes active effort to think about what your work or result or roadblock means for others, present options on what they should do, and you can tell easily when someone has thought through that next step, rather than just regurgitating what they've been working on all week.

That is a highly valuable skill and you know it when you encounter it.