One thing I miss from archiva is the ability to access a maven 1 legacy repository through nexus, as if it has maven 2 layout.
For example if you use JAXB, you need the Java Net repository http://download.java.net/maven/1
If you define it in your pom as having legacy layout, then everything is fine. However in enterprise environment, you might want to mange the repositories centraly, e.g. having <mirrorOf>*</mirrorOf> pointing to nexus, and define a single inhouse repository.
This works with archiva, but not in nexus!