v9.6 finally brought some parallel scans but is still limited in scope and far behind the commercial databases. Same with replication and failover, although there are decent 3rd party extensions to get it working.
I wasn't able to set it up properly some years ago and settled down with warm standby replica (streaming + WAL shipping) and manual failover. Automating it (and even automating the recovery when old master is back online and healthy) was certainly possible - and I got the overall idea how it should operate. But the effort required to set it up was just too big for me, so I decided it wasn't worth the hassle and settled down with "uh, if it dies, we'll get alerted and switch to the backup server manually" scenario.
Having something that goes in line of "you start a fresh server, tell it some other server address to peer with, and it does the magic (with exact guarantees and drawbacks noted in documentation)" would be really awesome. RethinkDB is just like that. PostgreSQL - at least 8.4 - wasn't, unless I've really missed something. I haven't yet checked newer versions' features in any detail, so not sure about 8.5/8.6.
It offers easy commands to perform failover and even has an option to configure automatic one.
After reading about github issues[1], I am a bit cautious about having automatic failover though.
[1] https://github.com/blog/1261-github-availability-this-week
As for automation... Things can always go wrong, sure. But I wonder how many times HA and automatic failover had saved the day at GitHub so no outside observers had a faintest idea there was something failing in there.
This is the one thing I ask of every database software, yet we still don't really have it. 90% of problems could be solved if there was a focus on the basics like easy startup, config and clustering.
SQL Server availability groups requires Windows Server Failover Clustering, which is not quick or easy.
Horizontal scalability is important for HA which every production environment would like or need.