As I mentioned in the comment above, the fact that one doesn't need to add anything in Maven to have eclipse compile the Kotlin classes but needs the maven dependency to run it, confused me.
In order to run a Maven project with a Kotlin nature, containing Java and Kotlin code in Eclipse + Kotlin plugin, it is necessary to add the runtime as a dependency in the pom.xml.
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-runtime</artifactId>
<version>${kotlin.version}</version>
</dependency>
<dependency>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-stdlib</artifactId>
<version>${kotlin.version}</version>
</dependency>
Incidentally it is also advisable to add the standard library since, in all likelyhood, classes from it will be needed by the code.
The second part is finding out the right version of Kotlin to use, which has to be the same used by the plugin.
For that, we can check the plugin version
The version of the compiler is already there (1.1.1 in this case), but we can also ensure this by going to the public Kotlin plugin source code repository.
In github select the correct branch. Probably "master" if the plugin is up to date. And check the Kotlin version in the pom.xml.
If that version is not in the maven central it will be necessary to add the pertinent repository both under the repositories and pluginRepositories sections in the pom.xml.
Don't remove the Kotlin library from the libraries tab in the classpath section of the project properties dialog (it's what appears in the image in my question above)
