I've often thought that a big part of the success of Twitter versus, say, RSS, is the consistency and completeness of the UI. Trying to find the RSS subscribe link, sending it to the right reader, and understanding the relationship between your reader and the site you came from is a fair number of hurdles. On the other hand, finding "follow" is easy and the behavior very well-defined. It sucks that Twitter is a big centralized service, but that's what makes it work so well.
Similarly, it's lame that every band is forced to have a MySpace page, especially given how ugly MySpace can sometimes be. But when I ended up at weezer.com trying to find a way to listen to the new album, the easiest thing to do was to find the MySpace link and click on it, because I know that MySpace = listen to album, and the UI for doing so is quite prominent.
I wonder if web designers will ever be able to agree on design conventions to overcome this tendency for centralized sites to be more usable?