This solves the problem of timeline being unreadable once you subscribe to enough people. Ain't nobody got time to read all that crap. Once everyone is rate-limited, everyone can easily digest their timeline. Without length limit, tweets become more thoughtful.
4. Fix the UI. Make it easy to view replies. Make it easy to view embedded images. Make it lean and fast. That would give Twitter advantage over similarly bloated services.
5. Anti-trolling measures. This one is really obvious! There should be no indication that you're blocked by another person, they just don't see you anymore. If the blocked person doesn't know they're blocked, they don't get the satisfaction of being blocked, and they don't know when they need to create another account to annoy you. This should be the basic rule when you implement a blocking feature.
6. Open up API. This one is obvious.