One could argue that the 2nd Ed. of Advanced Perl Programming is basically a CPAN guide on paper. ;-) And many articles on The Perl Journal, The Perl Review, and perl.com are also CPAN guides of some sort, although in the first two cases they require subscription.
I think such a site would be useful but, as others have said, the trick is producing the content. If you just create a blank site and expect people to contribute, you are likely to be disappointed. You are more likely to succeed if you seed the site with a good amount of content and make it easy to contribute. And you need to make sure that the site remains alive and kicking, even if you have to write the guides yourself. Otherwise the site may end up being forgotten, like the CPAN wiki at http://cpan.japh.org/ which hasn't had any update in months.
The listmania idea sounds interesting; it would be a good way of knowing which modules are more popular. There have been informal threads about it here (for example, "Desert island modules 472458), but it's much harder to count the results without the infrastructure.
