MapR deviates from the vanilla Hadoop & CDH distributions a bit. It keeps most of the services and structure (Job Tracker, Data Nodes, HBase Master & Region, MR, etc), but there are some significant differences.
One of the defining items about MapR's distribution is that it doesn't use HDFS. It has its own custom FS, which features HA and operates without Name Nodes (via distributed metadata). It also allowed them to enable NFS access years ahead of the rest of the Hadoop distros, as well as snap shotting.
The custom FS does complicate their distribution a bit, though ... for example, when you want to run products or services, you often need to install the MapR specific patches. When you want to run mahout, you need to compile it with the MapR patches from https://github.com/mapr/mahout. But it also gives them an opportunity to incorporate better security at the FS level, as seen by the implementation of "Access Control Expressions" and Cluster/Job/Volume ACLs.
Overall, it's a well structured product. My biggest concern is they've deviated so far from the norm that when new innovations are adopted, they're slow to adapt, because it has to be incorporated into their highly modified environment. YARN is a perfect example ... they haven't released it yet, even though their competitors have.
