[S]hortness is a good quality that shows the poster tried to condense his point into its true essence.
This is often true, but I think the value of brevity comes mostly from the context, a threaded comment page. In that context, users expect a discussion, not a monologue. They're not just looking to passively absorb information by reading, they're looking to sharpen their own understanding by actively articulating a response. That's harder to do when you're trying to respond to a comment that makes multiple points. Better to break a long-form comment into several more tightly focused short comments so that people can more quickly pick out the topics they want to respond to.
This is often true, but I think the value of brevity comes mostly from the context, a threaded comment page. In that context, users expect a discussion, not a monologue. They're not just looking to passively absorb information by reading, they're looking to sharpen their own understanding by actively articulating a response. That's harder to do when you're trying to respond to a comment that makes multiple points. Better to break a long-form comment into several more tightly focused short comments so that people can more quickly pick out the topics they want to respond to.