To summarize: An art critic possesses delicate sentiment, a quick and accurate perception of beauty. Delicacy comes through being practiced in the sense of having repeatedly experienced a certain kind of art as well as having repeatedly experienced the particular artwork judged. Experience facilitates the comparison of different works so that the critic acquires knowledge of the potential kind and range of qualities of those works. Moreover, a critic is free of prejudice that interferes with his adopting the right point of view. Finally, a critic has good sense: he employs his reason to overcome his prejudices, to make fine-grained distinctions among aesthetic properties, and to discern "complicated relations." This helps to ensure that his perception is discriminating and that his responses to art are appropriate. Together, these characteristics--the virtues of taste--distinguish accomplished art critics from persons of ordinary aesthetic sensibilities.[Tina Baceski, "Hume on Art Critics, Wise Men, and the Virtues of Taste", Hume Studies, Vol. 39, No. 2 (2013) p. 245.]
I've summarized Hume's account of good taste in these terms before:
(1) a broad base of relevant experience, so you can make informed comparisons;
(2) relevant skills of discernment, i.e., the acquired ability to identify important moves, novel twists, and the like;
(3) good sense, understood as the self-critical fairmindedness that allows us to be objective and unbiased.