The first ability people tend to lose after not playing is reading. That is, visualising a sequence of moves (without playing them) on the board and judging the positional value (similar to chess, but with astronomically more possibilities). Reading requires concentration, pattern matching (shape, tesuji, vital points), and guesswork.
After reading -- the tactical part -- some higher-level concepts get lost. Forcing plays (kikashi), inducing moves, estimating the value of thickness, obscure joseki, fuseki theory, and more.
There are /so/ many concepts and techniques to Go that unless you are actively studying and playing, it is nearly impossible to keep everything at the forefront of your mind.