It's the community!
The quality of a FOSS product is directly related to the quality of the community around the product. If there a product is well documented and has a group of active committers sysops and developers can solve theirs issues with the help from the community.
Commercial support contracts are merely a means for non-technical managers to cover their asses with higher management. Instead they should invest the money they now spend on support contracts in hiring well trained sysops and developers.