Have you seen this actually implemented somewhere? Note that I'm not contending that none of the short comments are valuable: far from it, sometimes they're the best thing in the thread. But having a limit would not necessarily deter that content from appearing. The authors would just share a bit more info/insight with us before hitting 'reply.' Meanwhile the timewasters really would be deterred. And while it's great to have sharp, witty remarks now and again, the problem is that loads of unfunny jokes appear, trying to emulate.
However, I suppose you could argue that valuable content would be blocked to some extent, it a knowledgeable contributor was put off because s/he didn't have time for a longer comment. Still, 50 words isn't much, or 30, 25... anything above the average sentence length I suppose would start to have the desired effect.
We implemented this on zootoo.com during a contest where posting rewarded the user with points (I understand 100% how ridiculous this is as a concept, how awful it is at achieving any kind of 'quality' by itself - and if I was the one who had been allowed to make the decision on this then it never would have been implemented - but karma is a kind of never-ending contest). Not only did the minimum length limit not work DURING the contest, but even after the contest was over and we had kept the comment length requirement in place, the quality of comments did not increase, and if someone wanted to say something that was below the minimum threshold they would simply add 'foobar-like' content to their post to meet the threshold.
The problem with time wasters is that it's no big deal for them, they're already wasting their time. It's always going to be up to users who care like you, me and others to try to mitigate the exposure of those time wasting comments while not penalizing legitimate contributions by users who care, and that's why I don't think a hard comment length should be implemented.
To be honest, you're probably right. Unless comment padding was universally down voted, there'd be lots of it going on, defeating the point. And sometimes valuable things like corrections are at loggerheads with a minimum word count.
I used to actively participate somewhere with a 10 character minimum. It caused people to add remarks like "10 chars" after a perfectly valid, meaningful short reply to get it past the filter. It also caused savvier members to come up with creative ways to fool the computer into accepting their shorter post without making it appear longer to readers, which caused less savvy members to wonder out loud if some folks had special posting privileges.
However, I suppose you could argue that valuable content would be blocked to some extent, it a knowledgeable contributor was put off because s/he didn't have time for a longer comment. Still, 50 words isn't much, or 30, 25... anything above the average sentence length I suppose would start to have the desired effect.