Oh come on.<p>I have your answer: because this is a nice controversial title to post a blog under.<p>Answering your specific points:<p>1) Timezones: there's people posting on Twitter 24/7, yet this is perhaps your only half-valid point... Yes, if you're in a different part of the world twitter might be hard. But there are plenty of people in your part of the world on Twitter - try following more of them, or accept that people in a different part of the world will be less likely to post in your timezone.<p>2) Tweetie and the Twitter website both support this. Click on "in reply to" in your Twitter feed.<p>3) That's what ReTweets are for.
Twitter is a bad place for conversations, that's kind of the point/charm of it. You fire and forget. You don't expect (or want) a response. It took off because it played to the ego of popular bloggers, then to the ego of corporations and celebrities.<p>If you prefer conversations to context-free statements, Twitter has never been interesting.
So what you want is Twitter to do exactly the same as the news feed of Facebook (which while it isn't perfect, it's the best social interaction framework I ever used).
I used Twitter quite a bit about a year ago, but stopped using it for the last several months. I went back recently to see what had changed and was really underwhelmed with what they've done.<p>Spam is what really stood out to me. The trending topics were littered with spam and I had bogus followers as well. I had hoped they would have cleaned things up a bit and added more than lists to the service.
1. The live stream is the point of Twitter. Notice how the major search engines made agreements with Twitter to get up-to-the-second results.<p>2. "in reply to"...right there on twitter.com, and any halfway decent client.<p>3. Retweets and favorites allow you to recommend/rate individual tweets.<p>EDIT: swombat already posted these points before me :/