Why does New York Times online require you to register before you can read the articles, while a lot of other papers don't?
The first answer that pops into your head might be that they want to keep track of their online readers. But that doesn't make sense. Other newspapers want to keep track of readers too. So do companies, blogs, and basically every other voice out there that wants to get heard. They don't require registration.
Besides, today web technology is advanced enough that looking at unique page visits serves the same purpose. It's much easier, both for the writers (who don't have to deal with user information) and the readers (who don't have to deal with interruptions).
A more realistic answer is that NYT requires registration because they want to give the illusion of exclusivity. If a hundred newspapers don't, and NYT does, people automatically assume it has something better to offer. Something that will make them want to be interrupted, to take the time to fill out a form and give their personal information.
I'm not sure if it's worth it though. People are not stupid. They know that if they want breaking news, they can get it from CNN.com or MSNBC or Google News, none of which require registration. If they want reviews they can again use Google. If they want classifieds they can use Craigslist. But if they really want to read NYT's news articles or editorials, they won't register online. They'll go buy the paper.
In the old days people would feel privileged if they had access to exclusive, prestigious sources of information like NYT. Today, it's the source who should feel privileged for having people choose them over thousands of others. And requiring registration is not a very good way of thanking your readers for their time and attention, both of which are limited.