Twitter does not display new lines. And that’s it – you can’t configure it to do so, you are stuck with no new lines. And I know this is a minor thing, but these things stack up.
Why do you need new lines? If you are tweeting a short poem. Or a dialog (e.g. a joke with a dialog). Yes, these are not the majority of tweets, but they are not that few either.
In fact, twitter stores the newline information and serves it through the API. That’s why most (to-be-killed-soon) 3rd party clients properly display new lines.
But this can be abused, you say – someone may enter 20 new lines and thus break the timelines of their followers. That’s an easy thing to fix – just don’t display more than 2 consecutive new lines, and also limit the total number of new lines per tweet to, say 5 or 6.
<rant>But no, twitter will enforce their rules and kill other clients, where you can properly read poetry and jokes, but I don’t think they will fix their own client.</rant>
At least make it configurable.