Rules about what apps can and can't do.
At first Twitter had a hands-off policy with regard to apps. But that's changing, radically. Read the rules, and see how many of them have anything to do with technology. Now, recall that when Apple first started with the rules, they said they needed to control things because of the technology of phones was different than desktop computers. Clearly there are major motivations that go beyond protecting users in what platform vendors do to control what kind of software and content flows over their networks.
erichippeau: "There should be a rule on Twitter that no one can link to a page behind a paywall. Useless."
If Twitter wanted to, tomorrow, they could block all links that went into a paywall. That would either be the end of paywalls, or the end of using Twitter as a way to distribute links to articles behind a paywall, which is basically the same thing, imho.
Twitter already has rules about what you can point to from a tweet, and they're good ones, they keep phishing attacks out of the Twitter community, and they keep out spammers. But that does not have to be the end of it. And if you think Twitter depends on you, I bet Adobe felt that Apple depended on them too, at one point.