"Features" like giving bogus error messages and destroying drives with Disk Doctor?
Seriously, NU 7 is a real POS. Drive 10 has come a long way. Version 1.1.2 is very good.
But I voted for DiskWarrior, primarily because PlusOptimizer (included on the CD) is the safest defragmenting utility available. Both Norton Speed Disk and Drive 10's defragger are vulnerable to power fluctuations that can severely damage your disk.