"Reasonable difficulty level" is not something objective. It depends greatly on who plays the game. Whether the game looks good is as subjective add judging painting, whether it sounds good as judging music. One can dislike game look and it can spoil his experience the same way painting style can.

There is objective quality in painting as well, such as the proper application of technique. And those things you mention about games seem fully subjective to me. A more objective property would be "fails to run" or "crashes often", which cannot happen to a painting.

If you find your self using the words "feels good" in a sentence you are very much on the subjective side of things. Likewise terms like "generally works", "runs well" and "reasonable difficulty" will all mean different things to different people. Objective items might be things like uses 4xMSAA, is presented in the first person and uses 'X' to jump. Even still once you consider how these choices effect the game quality you are straight back to subjectivity.

Painting can equally be criticised on technical merit, and as with games that can still be very subjective even though many would like to pretend otherwise.