For me, it's simply the overall feel of the game: it resonates at a perfect pitch for me, and I love the mix of an MMORPG with the world of the Elder Scrolls games, that I so dearly cherish. The landscapes, the aesthetics, the balance between questing/solo content, and group-based and more social stuff, and an unmitigated masterpiece of a soundtrack that I absolutely adore all make immersion and enjoyment very easy for me, with regards to raw play-ability. I find that ESO is quite rewarding - the pleasure of building new characters and feeling their power progressively grow - and there's little more satisfying in ESO for me than gradually improving my characters, and reaping the rewards from that, (becoming more and more successful in their intended role, for example, and a longing to improve being instated by successes). Often, my driving force is a desire to improve my characters, and the meticulous building of the four little guys sat on my account is something that I enjoy immensely, and find that I can really get my teeth stuck into.

I also really like the game-play, and once I get turned off something because of over-playing it or frustration when something isn't going as I intended it to, there's always another thing I can turn to. Whether it be PvP, (despite its flaws, I still greatly enjoy battles on the fields/bridges of Cyrodiil), dungeons, trials, exploration, questing - heck, even sometimes grinding - I find that there's always something that I can do, and I don't even own any of the DLCs. The plenitude of content in merely the vanilla game is something that I love, and I also greatly enjoy the social element of ESO. I've met some genuinely lovely people in-game, and have been acquainted with many awesome folks in my brief time spent on these forums, and, for me, ESO just does it all. Sure, the game might have its flaws - very little is perfect in this existence, (if anything) - but ESO resonates with me perfectly. I've been playing for roughly 3/4 of a year, and have loved every single moment of it; I'm still yet to experience 80% of the base game, and I eagerly anticipate that venture.

Also, I'm a wee, tinsy, tiny bit of a massive nerd, but we can let that slide...

For me it has to be playing with people.

Even today, a guild member asked if anyone wanted to go into Imperial City, a small group of 3 of us went. Had a great time ploughing through the sewers whilst fighting monsters and players alike, all whilst talking on teamspeak

The virtual world.

That they sell us broken content and leave it broken for months, sometimes years.

The enormous scope of the game combined with top notch world building, visuals (PC, high), architecture, lore, music, writing, voice acting and subtle details everywhere if you pay attention. I played countless hours since I started playing last October and I've only covered all the DC zones + Wrothgar + craglorn, with everything else to discover.

Living in the TES world. The players make it a new adventure every day.

PVP

The elusive recipe completionist desire... so many still yet to find and I refuse to purchase them!