Been playing a bit the past few days. I loved Skyrim and Oblivion, the work that Bethesda puts into the ES games is so deep, and has so much attention to detail. When you think of the size of the map, the books, the lore, the storylines, etc, it is incredible. So far I've only gotten to ~level 15 on ESO, and haven't even bothered trying any other player interaction, from trading to partying up for quests.
The game isn't fueled by microtransactions either, which is a nice change from the trend of games nowadays, though $15 a month is a very steep price. There is a solid money sink in the game to keep the currency valuable (Asians were buying for $1/k early this weekend and selling for $2/k) through item repair and travel costs.
One issue that is most important in multiplayer games is the need for solid balancing. There are several different classes, which the only difference between them is the abilities that they can use - no gear or storyline restrictions. One problem with balancing that I was hoping that wouldn't occur is with unlockable skills. When you level up, you receive an attribute point which can be spent on Magicka, Stamina, or Health, and a Skill point that can be spent on combat or crafting skills. The problem is that combat and crafting spend the same points.