cjk - Add CJK support for InnoDB fulltext search using app-text/mecab
client-libs - Build the client libraries from the server package instead of the C Connector packages (not recommended)
cracklib - Support for cracklib strong password checking
debug - Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
experimental - Build experimental features aka "rapid" plugins
jemalloc - Use dev-libs/jemalloc for memory management