It's been a few years since this became a trend, but there's some idiotic implementations founded on it.
the idea: throw unrelated words in a huge blob/cloud and change the size of each link based on how popular/important it is based on user activity.
the problem: it's still a huge blob of unintuitive, unrelated text.
example: http://digg.com/news/upcoming/cloud/comments, http://www.techpowerup.com/tags.php
user driven content:
the idea: let users influence what information and where that information is displayed on a page.
the problem: user driven content fails with extremely general information because there will always be a majority of a particular personality type that will influence the presentation and this type of person will dominate the presentation of your site.
example: google search
7 of the top 10 results for "velociraptor" are for a computer harddrive with the same name and NOT the dinosaur. Geeks live on the internet, they influence search results, yet do they represent most people's interests?
Wikipedia is commonly in the top 3 results (in my experience) yet I hardly want an encyclopedia article about my search topic.
Many times when I want to research something that is sellable, mega-sites that cater to anything that is sold are in the top 10 search results and yet their page for what I'm interested has only the product name and a picture (useless information).
Am I being pessimistic about this? User driven content and it's presentation is touted as a "democratic" extension of the internet's freedom of information, but the internet's majority of personality types and content producers hardly represent the interests of the majority of people in the real world.
It's like letting a millionaire be editor for a magazine that's meant to be read by people in poverty. Geeks, web site creators, and anyone who spends the time to cast their vote for a piece of information on the internet is intrinsically different than someone who just wants to access the information. The single editor who controls content and presentation with their livelihood at stake has been replaced by a slim majority of people who have no responsibility or stake in the quality of the content.
Web authors: use user driven content in appropriate places, and ask yourself whether the users who will drive your content really represent the users who will access your content.