Among the features coming to Xen 4.2 include support for Intel's Supervisor Mode Execution Protection (security improvement), allowing up to 256 host CPUs for a 64-bit hypervisor, supporting up to 5TB of host memory for the 64-bit hypervisor, multiple PCI segment support, PCI pass-through support for Linux guests, AMD SVM DecodeAssist support, Remus memory image compression, EFI boot support for 64-bit hypervisor hosts, XL tool-stack improvements leading to deprecating XEND, documentation improvements, and APEI support.
Xen 4.2 also drops the IA64 port, supports building with LLVM's Clang compiler, Xentrace improvements, and bug-fixing all-around. The Linux guest support for PCI pass-through is nice in particular for Xen 4.2 as it leads to some interesting possibilities; Phoronix benchmarks will be conducted soon. Overall though Xen 4.2 is looking like it's shaping up to be a nice release.
Right now there's weekly release candidates of Xen 4.2 being put out until the final release is ready in the very near future.
More of the Xen 4.2 features are detailed on the Xen Wiki. The release plan was mentioned on the Xen blog.