You can say a lot on 140 chars, also if something's junk you know quickly to not read it. It also doesn't take a lot of screen room, thus there's a lot of data available without a scroll down.

It would be interesting to have a 140, 700, 5000, unlimited character filters in Buzz. If its display algorithm was used well, it could work out if long posts from a particular person were usually not worth reading to most people.