All three of these are valid: 1. X is just an eccentricity --> Noun (countable) 2. X is just eccentricity --> Noun (uncountable) 3. X is just eccentric --> A

All three of these are valid: 1. X is just an eccentricity --> Noun (countable) 2. X is just eccentricity --> Noun (uncountable) 3. X is just eccentric --> Adjective For (2) see: https://en.wikipedia.org/wiki/Mass_noun https://en.wikipedia.org/wiki/Mass_noun (1) is probably the best in context here, i.e. "an eccentricity of the UK legal system"