Well, I am seeing a plus in the posting. Had 2-3 youngsters who liked to post something like "me too" and still posted a lot. Deleting all those messages and telling them yet again they should make better posts was a pain. Since the implementation they found out they do have more words in their vocabulary. I have a notice over the quick reply too, reminding them to post on topic and respect the rules (have the rules link there too). Don't know about others, but my members now put more thought in their messages. Not to mention that the automated warning is less personal and doesn't offend them at all. And it's not the end of the world: 75 characters is 75% of a line of a post (10-15 words) don't think it's quite that hard.
Hmm ... 75 characters is a lot for an Internet forum. While your posts might be of a higher quality, have you noticed a decline in new posts per day? It's possible many people might type a quick reply, see it not go through, so just say "the heck with this" and never bother to elaborate on it.
I have a minimum character limit set to 5 on my forum. I do get complaints but most people don't complain. It's something everyone got use to and its only 5 characters so its not like they can't just use a second word :)