* be very good at working with large datasets with computational tools (hadoop is an example)

* be a decent programmer, scripter, and hacker

* have a decent background in statistics

A good data scientist:

* has a good intuition and business sense

* can explain insights to non-technical people (usually through visualization and plotting)

* knows machine learning and predictive analytics

It's a vague term, but purposefully so. There's tons of stuff you can do with data, a data scientist knows what to do and how to do it.