Ok, the title should really be Solaris 11.4 Public Beta. However Solaris 11.4 is essentially what Solaris 12 was supposed to be. See the announcement and What's New document.
Alan Coopersmith created an excellent page with all different blogs about new features in 11.4 beta.
Also, see some blogs about sepecific features.
reflink(3c) What is it? Why do I care? And how can I use it?
Application Sandboxing in Oracle Solaris 11.4
Protecting Sensitive Data in Oracle Solaris 11.4
Immutable Zones: SMF changes & Trusted Path services
Getting Data Out of StatsStore
Where Did The 32-Bit Linkers Go?
Weak Filters: Dealing With libc Refactoring Over The Years
Goodbye (And Good Riddance) to -mt -and -D_REENTRANT
New CRT Objects. (Or: What Are CRT objects?)
Regular Expression and Glob Matching for Mapfiles
ld -ztype, and Kernel Modules That Know What They Are
ELF Section Compression
ELF Program Header Names
Core File Enhancements for elfdump
kldd: ldd Style Analysis For Solaris Kernel Modules
Default Memory Allocator Security Protections using Silicon Secured Memory (SSM ADI)
More adventures in Software FMA
Live Zone Reconfiguration for Dataset Resources
Migrating from IPF to Packet Filter in Solaris 11.4
What's in a uname ?