I personally feel that Twitter is unsufficient when you actually have something to say, for me it's no more than status updates, some news or nifty one-liners. I hate to write, or read, when you don't have anything to say and Twitter's signal to noise ratio is too great for me to use it. Instead I read and write blogs.
This discussion could go on and on, and it strays from the subject at hand, but I'll just say this:
If you feel that there is too much noise in your Twitter stream, then you're following the wrong people. You have full control over who and what you follow, that's the beauty of it. I've found that in general, following any more than about 150 people will get overwhelming. Find the 150 people that are most relevant to your interests and your lifestyle and follow them.
If you're "following" 2000 people, you're actually following no one.
I think the length is indeed a limitation for the person tweeting, but it gives benefit to the people receiving: short and sweet. It forces you to cut down to the point.
A twitter stream therefore feels like light reading. Low commitment. I suspect if twitter ever increased the length, their usage would go down.