There are a couple of well-rehearsed ideas that come up when you say Twitter's 140-character limit is a bad idea.
The limit enforces brevity and makes people better writers.
If they didn't have the limit it wouldn't work with SMS.
Here, once and for all, are the rebuttals to these ideas.
If you want quality writing, don't look to Twitter. Most of what you read there is crap. Unfunny humor, put-downs, highly abbreviated rallying cries that no one rallies to. Pathetic stuff really. If the goal of Twitter was to promote quality writing, then Twitter is a failure.
The best guarantor of quality is your ability to unfollow. If someone is boring you, or offending you, just unfollow them. If that doesn't do it, mute them. And in true emergencies you can block them. You have all the tools, without the 140-char limit, to keep the too-long writing out of your feed.
If you like SMS, great. You'll get the first 140 and a link to the rest. Which is more or less exactly how it works today, for everyone, even people who never use SMS.
If the 140-char limit is such a great thing, why do you see posts like this on Twitter??