The announcement of Linux 3.0 comes from the founder Linus Torvalds himself as he mailed it to the Linux Kernel Mailing list.
“As already mentioned several times, there are no special landmark features or incompatibilities related to the version number change, it’s simply a way to drop an inconvenient numbering system in honor of twenty years of Linux. In fact, the 3.0 merge window was calmer than most, and apart from some excitement from RCU I’d have called it really smooth.”
Beyond the numbering scheme change, this kernel includes POSIX alarm timer support, a just-in-time compiler for BPF packet filters, a new sendmmsg() system call, ICMP sockets, the merging of the Xen backend driver (completing the long process of getting Xen Dom0 support into the kernel), namespace file descriptors, and more.
Other New Features in Linul Kernel 3.0
See the KernelNewbies 3.0 page for lots of details.