I personally find the use of single word "This." in reply to a comment on forums (such as e.g. HN) really annoying. (But then again, I've reached an age where you tend to find everything new awful.) I guess the reason why it pisses me off is that it doesn't do anything that "Yes.", "Exactly.", "I agree" couldn't already do, but at the same time it has this overly sweet smell of kool-aid that just makes you sick.
For me (I’m 46 so I think I’ve reached “get off my lawn” age), it’s short for “this [is the correct answer.]” So I’ve never had a problem with it and probably used it once or twice myself. A couple summers ago at the pool I heard a teenage girl exclaim “poned!” after besting her friend at something they were playing. It took me a moment to realize she’d just said “pwned” since I’d never heard it said out loud before…
That pronunciation was popularized by the famous South Park episode on World of Warcraft [1] [2] which first aired in 2006. So it's not actually particularly new at all.
[1] https://www.youtube.com/watch?v=KPx__G9GkRY [2] https://en.wikipedia.org/wiki/Make_Love,_Not_Warcraft