"This" as the start of a reply is shorthand for "the comment I'm replying to makes a very good point that I agree with entirely" (and usually "and here's my tak

"This" as the start of a reply is shorthand for "the comment I'm replying to makes a very good point that I agree with entirely" (and usually "and here's my take on what (s)he wrote"), rather than "what you are about to read is the answer you are looking for".