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.
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.