What a truly awesome thing.
100Mbit/sec connection with client class NICs, 11MB/sec average throughput.
Not a lick of overhead either. Automagic mapping of IDs with the idmapd (the ID mapping Daemon) and recovery (sort of journalling) stuffs built in for less than perfect networking setups (ie: WAN or Frame Relay, etc...).
NFSv4 has better support for a wider array of client types. Works better with NIS and NIS+. Work beautifully with a well worked PAM setup. Works very well with a proper LDAP setup.
Very few things are wrong with NFSv4. About the only thing I can hit on it for is still the forever present problem of locking over NFS and a need for YAND (Yet Another Networking Daemon). Though it is no worse than the ID Mapping solution, just that locking is something that should have been worked out and not be causing locking issues.