Hi Michael,
> I have heard that you can now commit on the standby side when the redo data is in memory (instead of writing it to disk first)
Yes this is true and called "Fast Sync" nowadays. Official 12c R1 Documentation "1.5.6.3 Fast Sync":
Data Guard maximum availability supports the use of the
NOAFFIRMredo transport attribute. A standby database returns receipt acknowledgment to its primary database as soon as redo is received in memory. The standby database does not wait for the Remote File Server (RFS) to write to a standby redo log file.
However this attribute was also available (and working) in the previous Oracle releases, but you needed to set it manually. Oracle 11g R2 Documentation: AFFIRM and NOAFFIRM
> I cannot identify a real killer feature on 12c so far, they seem to have just steadily improved what was there in 11g
In my opinion there are two "killer features" in the Data Guard section. Cascading standby database support for DGMGRL and Oracle "Active Data Guard Far Sync" (in combination). Now you are able to build up SYNC DG scenarios over long distances without a huge performance impact and manual interaction (like the need of a "full cascading standby database") or third party (WAN-)accelerators.
Multitenant databases can be very useful in SAP environments (just think about the time for Oracle upgrade cycles in large SAP system landscapes), but as always it depends on SAP and their will to use all of that stuff. I totally trust in Dr. Christian Graf and his team to identify such benefits and use that features in the future.
Rock on 
Regards
Stefan