NSD is being developed with the purpose of creating more diversity in the DNS landscape. Secondly we want to create a fast, modern, RFC compliant nameserver.
The differences in responses on the wire between BIND and NSD have been investigated, see pdf. There are no interoperability problems for resolvers.
For NSD2 versus BIND8 an older comparison document exists, DIFFERENCES.
Further documentation can be found in the source code's documentation directory