Mesos made a ton of new contributions to distributed resource management. Resource offers and application integration can allow high cluster utilization and efficiency. Giving applications the opportunity to take part in resource allocation and scheduling was similar to the Exokernel design, and led to many interesting middleware architectures.

Mesos also introduced Dominant Resource Fairness for allocating resources to competing applications, which has become an important concept in CS and specifically computational economics in its own right.

Mesos achieved the rare combination of using CS research towards a widely deployed operational system. Its C++ code is simple enough to understand and hack, and seemed like one of those projects that you can/want to reimplement on your own "in a few days".

It is sad how quickly it has become obsolete.