This paper might seem short, but it's a great introduction to various scheduler types available for Hadoop clusters. Fair Scheduler & Capacity Scheduler are the only ones I've witnessed in production use, but I see a huge potential for improvement in the longer term by using some of the adaptive types.

It's amazing how many enterprise customers encounter severe issues with their clusters simply because of poor scheduler configuration. Either they never changed the defaults, or they arbitrarily twisted knobs without understanding the how various configuration values are dependent on each other.

Finding a healthy balance between having humans describe what they think they want and letting the system adjust itself will be huge.