The whole concept is rather nebulous at best. I’m not sure if there is a concrete, agreed-upon definition yet. But to me, a Library 2.0 mentality means refocusing on patrons/users/customers/whatever you call them. With the possible exception of applying new technologies to a user focus, this really isn’t anything new. But user-focused planning and services are absolutely critical if a library wants to remain viable. Public, academic, private, any kind of library should work this way. And anything that puts a focus in that area, whether it be better web utilities or more ‘traditional’ services can’t be a bad idea.
To some, ‘Library 2.0’ conjures up only images of new computer and web-based services. That’s part of it, to be sure. But only as a means to and end. Opening up your catalog, adding custom RSS feeds for searches and the like serves the user. But evaluate the new services by existing criteria. Does it improve accessibility? Will users actually use it? etc. Don’t get distracted by the shiny new things if they’re not actually worthwhile.
Much has been said by others in the links above and elsewhere, so I’ll leave it at that.