For newcomers to map-reduce programming with R and Hadoop, Jeffrey's presentation includes a step-by-step example of computing flight times from air traffic data. The last few slides some advanced features: how to work directly with files in HDFS from R with the rhdfs package; and how to simulate a Hadoop cluster on the local machine (useful for development, testing and learning RHadoop). Jeffrey also mentions that the RHadoop tutorial is a good resource for new users.
You can find Jeffrey's slides embedded below, and a video of the presentation is also available. You might also want to check out Jeffrey's older presentation Big Data Step-by-Step for tips on setting up a compute environment with Hadoop and R.