I hit level 40 today! Yay for me!
I was bragging about this to my friend who has played Dominion just a few times (let me tell you, he was SO impressed...), and he asked me how much of being good at Dominion is memorization. He was comparing it to chess, where, he said (I have no idea), you can improve a lot by learning how certain openings score against others, essentially. (If you're familiar with chess, great, but let's not compare chess and dominion cuz that's silly.)
Clearly, knowledge helps a whole lot in Dominion: for instance, Jack is a much better opener than it appears to the uninitiated, wharf is really strong and versatile, cursers are (usually) a very high priority early etc. My sense is that most of this can be picked up just by playing, though familiarity with DS and F.DS, councilroom (RIP), etc speeds up the process.
So I would say that memorization alone could get you to level 20 or so, but because, unlike chess, the setup of dominion is different every time, you need the skill of seeing how the cards interact. Of course, some combos come up a lot, and learning about those might get you to 30 (maybe), but the kingdom will perturb that knowledge. I don't think there's any way to obtain that skill besides playing.
So, how much of being good at Dominion is memorizing stuff, do you think?