Having recently gone through this evaluation process, I can tell you the following factors lead to us adopting Designate:

1. A clean ReST API for managing zones and records
2. A command line interface
3. With incubation (TBD), tighter integration with other Openstack projects (e.g. Nova, Swift) will follow. For example, each new server provisioned via Nova can have an "A record" created automatically. Language bindings for common languages (php, java, ruby, python) will also likely follow.
4. Designate supports multiple authoritative name servers; there is even work underway to support multiple backends with a single deployment. An operator could feasibly run both BIND and PowerDNS at the same time for different zones if they chose to. This is useful if you have a large number of zones to manage as well or if you want to prevent "lock-in" to a specific name server.
5. Multi-tenancy allows you to host multiple projects/organizations and keep those organizations' resources secure.
6. Designate administrators can limit the number of zones & records a tenant can provision
7. A Debian package and Chef recipes are available for managing deployments
8. The Designate community is growing and healthy:
   
   • HP (project founder), eNovance and Rackspace joined this year. Other yet to be named companies are ramping up their developers.
   • There have been 33 commits to master since November.
   • A Designate "mini summit" is planned in January