I actually really like where RethinkDB is headed, and within the year most of my issues should be resolved.
Another couple databases to consider, depending on your needs would be ElasticSearch and Cassandra... it reallly depends on your use case.
It's wild how many options there are that tailor themselves to all kinds of data out there.
We do not plan to implement secondary unique indexes. The philosophy behind RethinkDB is that if a feature cannot be efficiently scaled across multiple nodes we don't add it, and unfortunately unique secondary indexes are one such feature.
AFAIK most NoSQL databases don't implement it, and a few that do take two approaches -- forbid it on multiple shards, or just take a massive performance hit during sharding.
We chose to keep the feature out of the database. This way the application can be architected to account for it, so it remains fast as the database scales up.
And we are actually considering ElasticSearch to deal with this querying performance issue.