I don't know if this has occurred to anyone else, but after playing with it a bit and discussing some of the cards, a secondary theme seems to be emerging.
Empires is the expansion that desperately tries to give you things you really don't want.
Seriously, almost every card gives you something that is actively bad.
Many card-shaped things give you debt.
Many card-shaped things give you (or encourage you to keep) copper (Banquet, Settlers), silver (Delve, Conquest, Rocks), Estates (Wild Hunt) or even curses (Ritual).
There's a $5 silver that does nothing to make your deck perform better (Plunder)
There's a $5 do-nothing cantrip (Groundskeeper).
There's a $5 Peddler-variant (Emporium) that's worse than all previous ones (Market, Treasury, Artificer, etc.)
There's an event that tries to seduce you into sabotaging your own beautiful deck-drawing engine by cramming it with Gold (Windfall. It's exactly the kind of card that a semi-experienced player says "Wow!" about and an expert says, "Hey, wait a minute...")
Of course, other than the debt and treasure-gaining, it's mostly about the VP.
One of the most elegant and clever design decisions in Dominion, present from the very beginning with just the base set, is that victory cards are bad. Fundamentally, when players buy victory cards, they damage their deck, which slows them down so that the opponent(s) can catch up. Too many games neglect putting in a balancing mechanism like that, so when a player starts winning, they accelerate (I'm looking at you, Catan.) It takes careful and deliberate planning to get around that mechanism in Dominion, which is part of what makes engine-building so rewarding.
So after years of expert players figuring out how to optimize engines to mitigate the damage done to decks by the basic process of trying to win, what does Donald do? He gives us a huge number of additional ways to damage our decks by providing unskippable alt+VP generation. Nicely done.
Not that this makes engines less viable. In many cases, it's just the opposite. Historically, I've frequently looked at a board with a beautiful drawing engine on it, but with absolutely no payload! How many times have you seen fantastic draw, trashing and coin, but no +buy, and no attacks and realized that the best-case engine is a boring Province-a-turn which might not even outrace BM? Empires changes that by providing a whole ton of payload cards that offer far more interesting choices and opportunities, but very little actual help drawing deck.
Donald could have succumbed to the power-creep temptation and just given us a bunch of cards that were wildly more powerful than previous cards (and he sure as heck threw us that bone with Donate and Fortune) but instead, he reminded us what the basic mechanism of the game was and said, "Hey! You kids quit running all over the house flinging cards everywhere. Sit down and eat your vegetables."
Empires is greener than a Kale and Spinach salad.
It's a great design decision, and really improves the game dramatically.
(Edited to list a few actual cards.)