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This page contains advice to ensure smooth transition to maven 3 from maven 2 :

A set of maven-enforcer-plugin rule

Check that plugin versions are defined
TODO: Ensure that plugin defs appear only once
TODO: More rules!

<project>  [...]
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-enforcer-plugin</artifactId>
        <version>1.0</version>
        <executions>
          <execution>
            <id>enforce-plugin-versions</id>
            <goals>
              <goal>enforce</goal>
            </goals>
            <configuration>
              <rules>
                <requirePluginVersions>
                   <message>Best Practice is to always define plugin versions!</message>
                   <banLatest>true</banLatest>
                   <banRelease>true</banRelease>
                   <banSnapshots>true</banSnapshots>
                   <phases>clean,deploy,site</phases>
                   <additionalPlugins>
                     <additionalPlugin>org.apache.maven.plugins:maven-eclipse-plugin</additionalPlugin>
                     <additionalPlugin>org.apache.maven.plugins:maven-reactor-plugin</additionalPlugin>
                   </additionalPlugins>
                </requirePluginVersions>
                <unCheckedPluginsList>org.apache.maven.plugins:maven-enforcer-plugin,org.apache.maven.plugins:maven-idea-plugin</unCheckedPluginsList>
              </rules>
            </configuration>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
  [...]

</project>

relative path to parent

If a project is referring to a parent pom, but that pom isn't reachable in the default parent path (../pom.xml). Then you should either :

update it to the right path
put <relativePath /> in the <parent> tag so that maven knows it will have to look for it directly in the local repository

Standard variables

For example, when one wants to refer to the artifactId of a POM, the variable ${project.artifactId} can be used.
There are a lot of other possibilities to do that, like ${pom.artifactId}, ${artifactId} and so on.
These ways are all deprecated and might be deleted in the future. With maven3, all these deprecated uses appear as warnings.

Always use the ${project.xxx} form

Thank JVZ for the page title: http://twitter.com/jvanzyl/status/4621533361