FogBugz offered a server version where you could install on a private server yourself. They still offered a cloud version if you didn't want to manage the data yourself.
This point is huge. I'm a technical writer for a large conglomerate, but often get pulled into drafting or contributing to business documentation that is all created in Word. These things have to cross the desks of over a dozen different people spread across three continents so versioning is painful and it's often left to me to put everything back together. I need this product, but there is no way we can put sensitive sales and IP details onto some random cloud service.
Make this a standalone product and it's a no-brainer that can command an enterprise price point.