I agree auth is a huge success, but no alternate DB support is a not starter for most I imagine. Any other non relational DB support would be a huge win, most people that I have talked to are blocked by Mongo's license.
For example, I worked at a public library for years, and the policy was no AGPL, even if we had no intention of touching the source. Same for a local university.
[Edit to add anecdote]
http://www.mongodb.org/display/DOCS/MongoDB+Commercial+Servi...