For me it's the overall style, quality and performance of the solo/co-operative PvE content, under-pinned by the TES lore which I've followed since Arena. I've played MMORPGs since 1998 and for me this one beats all the others, with my only criticism being the trading system.
There's a lot I could say why I like it, but it all boils down for it just feeling fun and great to play, despite its flaws. I've also played many MMORPGs over the years and if I'd ever played ESO before, I wouldn't be able to go back to any of them as seriously as I like this game.
The atmosphere of the game in general is awesome though, not just the atmosphere related to TES but the graphics/feel in general.
I love the lore, the details of the landscape, housing, the community... thats what keeps me playing.
What I am very puzzled about is the quality of zone chat since the launch of Elsweyr.
The Elsweyr zone chat is full of hate and aggressiveness, as well as primitive comments. I am not used to that. ESO has always better than that. It has been three times or more already only today, that someone posted in zone chat in another language than english (russian, german...) and got BASHED for it.
I very strongly hope that those trolls quit playing soon again.
What a great question. I'm mostly in love with trading, but I also enjoy housing and farming. I'd say my least favorite is whenever I have to venture into pvp land for quests on holidays or something.