I'm 99% sure this is to encourage people to take discussion to the discussion part of the site, which makes sense. But sometimes people ask questions in comments, and if I want an answer that is longer than a few sentences, I have to do it over several comments. And if I want to post a link that's a bit long, it is very hard to put a good explanation in with it, and I really don't want to separate the explanation of the link and the link itself. And if I want to @reply to another user, I have even less room.
For example, this comes out to exactly 140 characters (when you add the colon after the http)
http//meta.programmers.stackexchange.com/questions/171/whats-the-difference-between-programmers-and-stack-overflow There's almost no room left, help!
I know that discussion doesn't belong in the comments, but in this case, what are the comments even there for? It would be silly IMHO to get rid of them and make everyone ask a question on the discussion site if they ever wanted to say something. 600 characters like the other sites would probably be too many, but I don't see why a 200 char limit instead of 140 would hurt too much, and it would really help this issue. :D
EDIT: this has been discussed in this question already. But I'm still not sure what I should have done in the situation on the linked to proposal where a user asked the difference between programers and area51. Yes I could have just left the link with no explanation, but I wanted to say a bit more for clarity's sake. Should I have created a discussion question on the proposal that asked his question on it for him, posted an answer, and commented with a link to that question? This doesn't seem right, but I understand that we have to start transition to the new discussion zone somehow.
Is this the way it should be done? I could just refuse to answer all question in comments and only insist that I'll give them an answer when they actually create a question on the discussion site. It doesn't seem right, but if it is the best way to handle the situation I'll gladly start doing it. :D