In this stimulating interview on InfoQ, which is one of Sadek Drobi's better interviews, Erik explores the dualistic database design space along 3(+1) dimensions.

http://www.infoq.com/interviews/meijer-big-data

There's a pretty explosive growth in new database engines, so this talk is highly relevant. Just Google for "new databases"  and you'll find a nice diagram.

Erik also hints at interesting ideas along the way.