I think it isn't a bad idea, even though sometimes one wants to express a simple word of appreciation under a post overloaded with comments — nice! great! breathtaking! I agree, I totally agree! … — and the author would be flattered even if already read 99 identical comments… or maybe it isn't addressed to the author of the parent post but to the author of a comment…
It could become a race to write the new shortest sentence which expresses the same “concept” already expressed but in a different way.
It's a feature that should be tested on the road…
It's been used elsewhere, like at 4chan and at Xkcd's boards.
You already
toldimplied that (it is used by xkcd) in the post. But each community has its flavour and may react differently to constraints, even if well motivated and reasonable.