An example of the sort of problem that developers currently encounter can be seen at http://blog.jonasbandi.net/2008/12/using-jpa-and-hibernate-with-maven.html. I have encountered similar problems in the past, e.g. trying to decide which version of a plugin is the current one or being unaware that an artifact I was using in a build had been superseded by another with a different groupId and artifactId.
I feel that Maven lacks a deprecation mechanism that would make it obvious to everyone that they're using something out of date. The problem is that once an artifact (other than a snapshot) has been published, it is never supposed to change thereafter. But it shouldn't be impossible to devise a mechanism for adding deprecation and redirection to the associated metadata. It would then be possible for the dependency resolver to display a warning whenever a deprecated artifact was used in a build, either as a plugin or as a library.