Sorry, whenever I see someone start a comment or post with "This ^^^" or the like, I always read it with attitude, and that colored the rest of the post for me - that was my bad. Have a nice day, fellow internet commenter.
Oh. And I always thought, it meant, that someone wholeheartedly agrees with something. A bit similar like saying "dito". Or maybe "same", "I agree" or probably "well said". Best fitting might be the old "hear, hear".
If there is any attitude would depend on the thing "this-ed". Maybe people often like to use that type of thread slang to refer to negative remarks. If so, I failed to realize it.