Among the many changes for QEMU 2.0 are ARM emulation improvements, many x86 emulation improvements too, Xen PCI pass-through with a ROM now works, USB device improvements, the Tiny Code Generator (TCG) has its optimizer generating faster code and supporting new instructions, LTTng 2.x is supported for tracing, block device improvements, and a whole lot more. A rough change-log for QEMU 2.0 can be found via the QEMU Wiki.
The current plan is to have three more release candidates in the next few weeks leading up to the official QEMU 2.0.0 release on 4 April. It's hoped that QEMU 2.0 will make it into Ubuntu 14.04 LTS for its release next month.
More details on QEMU 2.0.0-rc0 can be found via the mailing list release announcement.