Fear of Upgrades
Lack of BackwardCompatibility between different version of the same library, framework or system can result in a situation when users of such technology refuse to upgrade to new versions, in a fear that the new version can destabilize the existing system.
Many of Linux notebook users know the feeling: after many sleepless nights, their notebook is sort of working. It can hibernate (and also de-hibernate), however it cannot suspend (and wake up). Very likely this is due to interaction with a graphic video card. A new version of X server is available: Shall I risk the upgrade? It may repair the suspend, yet it may also not help at all and potentially break already working hibernate!
I am constantly facing this dilemma since 2006 and it is not getting much better. Whenever a new version of my Linux is available, I need to fight with myself to convince me that the benefits of new functionality out-weight all the problems associated with an upgrade.
If this kind of fear appears among users of your library, then I claim that your library is not example of Good Technology.