A good Word press plugin must have the following characteristics:
Straightforward and easy install procedure: Installation is the main process in the implementation of a plugin especially for a system such as WordPress. WordPress has a lot of features already built in and the plugins are developed to complement these features and add custom features for the easy administration and manipulation. If the installation of this system is complicated the administrator won’t go for it.
Straightforward un-installation process: Un-installation is as important thing for a plugin as the installation process. Un-installation gives the administrator the power to decide when to remove the plugin when it has no more function to perform, no administrator would want a plugin that cannot be removed and remain there forever.
Easy upgrade process: A good plugin must have a good upgrade process, a plugin must be able to adapt to situations and when a new feature can be added, it should be added without altering the whole system. Updates are required for all kinds of software, it is even more important for a software component such as a plugin.
Good localization and language support: Language support is very important as it makes sure that any administrator who is not so fluent with the English language too can access and modify whatever he/she want to.
User Understandable errors: A good plugin must throw exceptions and errors that are understood by the end user to rectify and correct it if possible. Errors can be a very big problem if it is untraceable. There must be a system by which the administrator should be able to contact the developer to get all the details without actually getting confused or giving misinformation.
Import and Export options: There has to be an import and export option for any software plugin like the ones in WordPress, this would enable the administrator while porting the whole system to someplace else or while changing the back end.
Intuitive user interface: A good interface is required to make the administrator work on the system easily. A good interface is a paramount requirement for a good WordPress plugin.
Security of the Plugins: A great plugin should not be vulnerable in the system.
Keeping the records of the changes made: A good change log is necessary to keep track of changes.
Good support: A plugin that is well supported and documented is a big plus.
The above mentioned are the most important characteristics of a WordPress plugin.