Twitters main problem is that it's a protocol more than an actual content platform. What they need is to get more content onto their platform and the only way to really do that is to allow for richers posts which means loose the 140 character limit.

I know a lot of people will say "but thats what so great about twitter" and I would claim that it isn't, they just think it is. The short tweets and the asyncronous character of twitter creates quite a lot of noise and so you are actually reading way way more than interesting 140 character tweets you also read a lot of noise and redundancy.

No whats really great about twitter is that it's more centerede around interests than friendship and that this provide the actual value of using the service.