Details are provided at http://www.europe.redhat.com/products/rhel/server/details/
Some important ones are:
1. Red Hat Enterprise Linux 6 supports more sockets, more cores, more threads, and more memory.
2. Memory pages with errors can be declared as "poisoned", and will be avoided.
3. The new default file system, ext4, is faster, more robust, and scales to 16TB.
4. Redhat Cluster Nodes can re-enable themselves after failure without administrative intervention using unfencing.
5. iSCSI partitions may be used as either root or boot filesystems.
6. The new System Security Services Daemon (SSSD) provides centralized access to identity and authentication resources, enables caching and offline support.
7. Dracut has been introduced as an replacement of mkinitrd.
7. XEN has been replaced with KVM.