There's so many that I'm excited about! I like additions that introduce unique interactions between players, encourage you to play drastically different decks, and/or change how really common things work, and there's a bunch of all that in Empires. In general, I'm way more excited for Empires than I remember being for previous sets (and I'm always excited). It may also help that I love alt VP, and this set has so much of that.
Cards:
Castles --> definitely interesting interactions between players and seems like it will foster divergent strategies (people don't benefit equally if they "go for castles" equally)
Villa --> the kind of card that makes me dream about all the crazy new things it can let me do. (Although the people I play with who already hate how long my turns can get playing an engine will probably be pretty unhappy about it.)
Split piles & Gathering cards --> unique interactions between players (especially if playing 3-4 player. I know, I know: heresy). I especially like Encampment (straightforward card, but can open interesting pile control tactics, and could encourage you to add cards you might not want in your engine if this is the only village), Gladiator (cool player interaction both with revealing a unique card from your hand and the opportunity/risk of making fortune available), Temple (without the gathering, I'd think this would be the kind of card you'd only buy one of. But if it's the only trasher, it's likely gonna get used a bunch, making buying another for points tempting. Gathering is a great way to make a trasher that involves new, interesting decisions).
Events:
Salt the Earth --> hilarious; player interaction; hilarious; great flavor; hahahaha, I'm going to relish how much this will make people hate me
Donate --> I agree with what everyone else has said about this, but I'm also interested what new golden decks this might enable
Delve&Wedding --> simple ways to make basic treasures feel new again
Tax--> same reasons everyone else has had
Landmarks:
Mountain Pass, Fountain, Keep, Obelisk, Wall...okay, all of 'em, really.
Thanks Donald X!!!