I guess what he mean was this: "Open core (a.k.a. proprietary relicensing[1]) is a business model where an open source product is also made available commercially with non-open-source additions" [1]

I cant speak to Riak, but generally this model can create a conflict of interest between the "enterprise features" on the one hand and open source commitments on the other. For example if someone submits code to the opensource version that duplicates/overlaps an "enterprise feature"

1: http://en.wikipedia.org/wiki/Open_core