One of the beauties about 140 characters is you have to think about what you want to say and how you say it. Editing for brevity often makes it punchier and better phrased. Many are the occasion when I have written a joke or insight for facebook, modified it for twitter, and then posted the twitter version in both cases because the twitter version was just better.
300 characters is not almost as quick to read it is quite clearly twice as long to read. For me, that would mean that instead of having time to follow 100 people on twitter I would only be able to follow 50. (Another of the other main features of twitter is allowing followers to interact with popular figures with their fans which I imagine also benefits greatly from a terser format.)
Personally, I think twitter has the potential to be one of the social media that survives in the long term. Yes it has obvious limitations but I get much more bang for my buck reading those condensed updates than I do using any other media platform. Sometimes when life gets busy I will quit other sites but twitter remains perfectly manageable and useful. It's great for news, politics, humor, and interacting with relatively large numbers of people. If you have an essay you need to share, you can link to it.
Some examples of how much you can pack into “It is the job of thinking people not to be on the side of the executioners.” -- Albert Camus
"Life can only be understood backwards; but it must be lived forwards." -- Søren Kierkegaard
"We are such stuff as dreams are made on, and our little life is rounded with a sleep." -- William Shakespeare