The final QEMU 2.1 release is expected next Tuesday (29 July) while issued today was 2.1-rc3. A final 2.1-rc4 release candidate is expected in the days to come for fixing one last critical issue with the release.
QEMU 2.1-rc3 just brings a lot of bug-fixing over the earlier release candidates. QEMU 2.1 overall has many new features like better 64-bit ARM support, improvements to Allwinner SoC emulation, KVM support for MIPS' Malta board, TCG improvements, various x86 enhancements, and full support for USB3 pass-through.
The QEMU 2.1-rc3 announcement can be read on QEMU-devel.