IMHO the main issue of Twitter is not even the way how it's now a walled garden (and since recently, effectively outside of the Web).
It's how the combination of a ~~140~~ 280† character limit, personal walls, hashtags, likes and an horribly hard to navigate comment system has resulted in a medium where constructive debate is almost impossible, while feelings-driven mobs rule.
†While moving to a 280 character limit probably made Twitter better, I'm wondering about the loss of compatibility with texts. One actually remarkable way in how Twitter was good is for post-disaster communication, which is the very situation where Internet might be down while basic cellphone communication still up.