Just tried the Red Hat Enterprise Linux (RHEL) 5.2 beta, for the server part, main improvement is virtualization part, new CPU features from AMD are supported, so a new version of hypervisor introduced for faster full virtualization.
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For client part which I use most, the great improvement is the upgrading of user programs, eg. pidgin, firefox, openoffice.org are all new version instead of old old "stable" version, which is a good news to normal users.