I would say:
Level 45+ players generally know what the cards are and the basic strategy for playing Dominion (value trashing, can build an obvious engine, know what attacks are good) but have major theoretical gaps. They often go for points cards too early, or overvalue landmarks such as museum or cursed shrine, especially early in the game.
Level 50+ players generally know what cards are good/bad on a particular board but make significant strategic mistakes (don't value trashing enough, go for points way too early, don't hit the right balance of action generating / draw cards, miss niche card interactions).
Level 55+ players (I am in this group, level 59) generally choose good openings and pick viable strategies, but often make significant tactical mistakes (don't switch from building to greening at the right time, miss endings for themselves or allow endings for opponents, don't hit quite the right balance between economy and draw).
Level 60+ players make very few tactical errors, almost never miss strong card interactions, and are very good at evaluating the relative merits of different possible strategies.