News people started paying attention early because Twitter was faster than anything else - I think due to the 140-character limit and, believe it or not, dumbphone support. When e.g. an earthquake happened, Twitter was where you'd see it first.
Once the news folks were paying attention some of them started reporting tweets directly and then it snowballed - there's a real positive feedback loop between news/politicians/celebrities - politicians and celebrities use twitter because the newspapers pay attention, and the newspapers pay attention because politicians and celebrities use it.
I think it's because they iterated really fast on new features, and presumably stayed with what was popular. Original Twitter isn't even all that close to what Twitter is today. gnusocial is playing catchup featurewise, moves much slower, and frankly the default theme is really ugly.
I assume they can see what uptake there is for features using analytics, but what comes to mind are native retweets and subtweets. Those are relatively new. They have also changed how blocking works multiple times based on feedback.
