A while ago, there was an online newspaper named the International Herald Tribune. Owned by the New York Times but published in Paris, it was quite a good paper, with fairly incisive articles not too far from Economist territory. Then someone at head office decided to kill the brand and roll it into the New York Times brand, and iht.com became global.nytimes.com. And, with that, inherited the New York Times' draconian insistence on users requiring to register and log in to view their their precious content.
The New York Times, you see, is not satisfied with the standard online news business model (make their content freely viewable and linkable and sell ads to those surfing in on web links from wherever in the world they may be); that may be good enough for rabble like their London namesake, but the NYTimes' content is worth more than that. At the start, they even tried charging for online access to it. Of course, as Clay Shirky points out, this is not a viable business model for online news (current events cannot be copyrighted or monopolised, and someone can always do it cheaper), so the NYTimes soon dropped the demands for subscription. However, they have doggedly kept the other part of the equation: the insistence on users subscribing, remembering yet another username and password, and giving a valid, verified email address, as well as some juicy demographic information. Of course, there are ways around this; the most popular site on BugMeNot, a website for sharing free usernames/passwords to such sites, is the New York Times. However, such accounts usually have a very short lifespan; either they perish when the email verification period lapses or, failing that, the Times' web admins hunt them down and kill them, like an ongoing game of Whack-a-Mole.
The New York Times, however, is not the most irritating example of coercive registration; that accolade would probably go to a site named, ironically, Get Satisfaction. This is an external tech support site, used by a number of web 2.0-ish sites, including SoundCloud and ping.fm. As a web site, it is the very model of a modern website; rounded corners, quirky retro fonts (oh so San-Francisco-via-Stockholm), pastel-hued gradients, animated fades, you name it, it ticks the box; it would be perfect, but for one fatal flaw in the human interface design.
What somebody neglected to notice is the typical use case of such a site. One doesn't go to Get Satisfaction to socialise with friends, share photos or music, find a date or a flat, or do anything one does on a typical social web site; one goes there when one has gone to such a site and found that it doesn't work properly, and wants to notify somebody to fix this. Now when that happens, the last thing one wants it to have to think up another username and password, and be cheerfully invited to fill in one's profile and choose a user icon representing one's personality. As far as support forums go, less should be more, and Get Satisfaction, for all of its pretentions to being some kind of online clubhouse, falls short.
Not everything that isn't charged for is without cost; there is a cost, in time and finite mental resources, to keeping track of usernames and passwords. (Of course, you could use the same password across all sites, but that replaces a psychological/time cost with the security risk of all one's passwords being compromised.) And sites which put registration speed bumps in their users' way could find users going elsewhere where offers a smoother ride.