Easy to learn software
Software should be easy to learn. This includes the configuration and the usage of the software.
Of course, “easy” is a relative and subjective term and it is not possible to skip or avoid some learning.
By “easy” we mean, that learning the software should behave forseeable and non-surprising for users with some basic knowledge. E.g. learning the editor
vi is hard for a new user, but a program which states it has “vi keybindings” should be easy to learn for users that have some experience with vi.
The same goes for other software. As we list often tools which are used on the console, users most likely have to learn some default keybindings - but the user should hot have to use dozens of different keybindings.
Configuration should be ideally happen in plain text files with clear instructions - as comment or clear names of variables and keys. Programs, which can only be configured by editing the source code, we don't consider “easy”.
A clear documentation of the program is mandatory, preferably as man page.