Already exist as "Continuous Archiving". Was done by third party tools in version 8, part of the core in 9. Details: http://www.postgresql.org/docs/current/static/continuous-arc...
> 4) Replication
Master - slave is in core of version 9. Allows fast switchover, but is somewhat harder to swith back.
> RAC
Logical replication is already in 9.4 . And with logica replication we might soon see full master - master replication in core postgres.
Incremental backups involve taking a full backup once in a while and then making interim backups of the changes since the base backup. WAL ( or archive logs in Oracle terminology) are still used for point in time recovery for changes that have occurred since the last incremental backup. One advantage is that if a row is updated 100 times between the base and incremental backup, you will only essentially be backing up the last change in your incremental backup versus WAL which will have all 100 changes. For big DBs, this can be very handy.
"somewhat harder to swith back"
Thats my point exactly. In Oracle switching between primary and standby is easy and you can do it back and forth as needed with no need to worry about rebuilding the database. In Postgres this is not yet the case, but most of the infrastructure is there already so its surprising it hasn't been done.
I'm also looking forward to logical replication. RAC is different however from master-master replication. Essentially its a specialized cluster with storage thats shared between all nodes in the cluster (so there is no replication going on) with "cache fusion" to share in-memory data between nodes .
PG lets the OS do the stuff it does best versus RAC which is largely a OS cluster solution specifically tailored for a multi node DB - so its unlikely (in my opinion) that the PG developers will ever produce anything similar.