E.g. the only way to communicate with someone without polluting your timeline (because who really cares what you @replied to someone) is DMing them, and even that is less convenient than even facebook messages. But no one uses DMs for most conversations so I have to go without following some awesome people just because they @reply massively and I have scroll fatigue already.
Another example – twitter's website has no new @mention notifications, and my SO accidentally discovered this feature when she downloaded Tweetie months after she started tweeting.
No wonder twitter's user retention rate sucks.