I'm not really going to go into depth, but I'm going to point out some of the biggest selling points for me, that screams "it's not a clone"
Character customization - 16+ skill trees, one for each weapon type, skill trees for race, things like vampires/werewolves, guilds (mages, fighters). An extensive character creation - I have yet to see one on a non-asian MMO at this point. WoW only has a few options for character creation.. swtor.. erm.. I don't need to explain right? You can technically unlock all perks in every skill tree under one character through skyshards. It's speculation, but considering the first three provinces take ~ 160-180 hours to complete, and you have to enter the other 6 to presumably get every single sky shard, you're looking at another 360 hours put into the game before you have everything.
A dedicated story line much like the TES series. Molag Bal is getting some much deserved love in this story line. I'm not expecting Morrowind-like quality for a main quest, but there's still a meaningful story TES fans will be able to pick up on. We know nothing about how they've been treating lore, so I'm not going to compliment them on that just yet.
Combat - there are two MMOs with action style combat, Neverwinter and Tera. Neverwinter was designed around a F2P model and isn't dedicated to the same level of content that can be pumped out like ESO. Tera wasn't at all committed to creating content, and didn't have depth to the game to begin with. It by far has the best combat I've seen in a MMO, but combat can't keep me interested for too long if everything falls short. Basically ESO feels like the first MMO with action-style combat that there's going to be a reason to play for an extensive time and for many years.
You are encouraged to explore - when is the last time you've played a MMO that makes you want to just wander around and see the beautiful world for what it is? Graphically wise, I can't vouch for ESO, but their same philosophy with single player games remains - there's going to be quests out in the middle of nowhere, and going into caves can trigger storylines. There's going to be a reason to get lost like you did in TES games, and it's going to be encouraged.