Not this. 
I think it is common enough to nullify points (1) and (2), such that if it really is a problem it is the ignorance of the reader that is the cause, not the exclusivity of the writer.
I don’t think it is lazy either. It doesn’t mean the same thing as “I agree”. “This” has a more extreme connotation. In pretty much all the cases where I have read it I have taken it to mean something to the effect of “I completely agree with this post, and it is a very well written statement that captures the heart of the issue at hand”. There are many statements I could read where “I agree” would be an appropriate response, but only a few of those could justify a “This”.
If that is laziness then my use of the word “laziness” is laziness, as I should have said “averse or disinclined to work, activity, or exertion; indolent” instead. This philosophy would make reading a Stephen King novel a life long project!
I would agree that a “This” by itself as a response is not terribly useful. It is better used as an introduction to an expansion of the original point, or an illuminating explanation of your agreement.
Like everything, it can used for good and bad. I do think it is a useful tool for communication a point clearly and quickly.