* immediate support (have someone you can call 24/7 if your system blows up)
* responsive support (have engineers on staff to close support tickets fairly quickly. This applies especially to bugfixes)
* good QA process so that you do not use your enterprise customers as free QA but pay people to test the product thoroughly before releasing it. This ties into the notion of long term support as something that stays in the QA matrix and receives bugfixes/updates.
* good documentation, in multiple languages
* A supporting ecosystem of certifications/training materials, so a business can hire people qualified to use the product
All of the above boils down to having people on staff doing all the boring, costly things that reduce the pain of companies using the software. Those things are not fun, so they tend not to happen consistently by open source volunteers, so they have to be paid for, hence "enterprise", as end users don't value these enough to pay for them over free but big businesses do.
