I sometimes use “This” as not merely a form of agreement but as emphasis - “This is exactly what I would also have said, and I am quoting it because it bears repeating.” It’s not just a “Me too”, it’s “Me REALLY too.”
“QFT” is much the same.
I first saw “This” in pit threads, and I thought for a while that it meant, “This is the kind of post we’re complaining about.”
I thought it stood for “Quit fucking trolling”, and usually, the contexts I’ve seen it in make perfect sense when viewed that way. Think about that the next time you feel like quoting someone and only adding “QFT”. They really did look like a troll. 
*** Ponder
Pineapple.
“QFT” has another connotation. On message boards that allow unlimited editing, quoting another is a way to capture what they originally said before they’ve gone back and changed it. The “QFT” means quoting someone to preserve the truth of what they said.
Obviously this version isn’t that useful here, but on less savory boards, it can be used to prevent trolls from changing their tune.
I see “IAWTC” a lot for “I Agree With This Comment” (oftentimes in blogs), or if it’s a really good post, “FTW,” meaning “For The Win,” as in, “This comment beats all other comments and wins.”
I am not, however, 50+ years old. Yet.
Esprix
FTW!