The flexibility to provide hook points into any step of the provisioning process as well as the ability to customize and extend reporting, dashboards and server tabs to provide additional information to the end user. No other product we've tested has such comprehensive capabilities natively built in. 5 minute upgrades are the norm. Five minor and one major update later the alphas are almost as rock solid as the GA releases.
Update: The flexibility and speed to deliver new features is definitely the best thing about this company.