Did you enjoy The Elder Scrolls series, a venerable list of RPGs from Bethesda Studios with a focus on open world exploration and quantity in gameplay? Well then you'll momentarily enjoy reminders of those games while you're playing The Elder Scrolls Online. You'll turn and recognise some architecture from Skyrim, or catch a reference to a guild from Daggerfall.
One of the two things that Elder Scrolls Online is a big pile of references. This means that, every time I play it I think about better games. Thanks to the Elder Scrolls Online I really want to install OpenMW. Now, you might say World of Warcraft was a big pile of references to its predecessors but WoW at the time was a new way of looking at the Warcraft world. It had a wow factor as it let you walk inside a Mage's Tower that in previous games had only been viewable from the outside. Elder Scrolls has done it all before. Some of it with less visual acuity, some with more. The only thing Online offers over those other games is the promise of multiplayer.
The other thing that the Elder Scrolls Online manages to construct is the perfect skinner box. The homogenization of areas is extreme with each map varying only in how the dungeons and points are laid out. They all have some dolmens, a few open dungeons, a group dungeon and a list of things to do.There's plenty of variation in the individual quests and the vistas can be compelling but you can't shake the feeling you're ticking off boxes on a checklist. That checklist is the achievement list. Even at the faction level each faction has a starter zone, then one main area for each level range. The DLCs are a welcome change of pace as they're their own areas, breaking out of the series of droning progression. They must have realised at some point what a drudge it would be because the One Tamriel update allows you to level anywhere as any alliance. You can hop over to the high level Dominion zone and quest there as a low level argonian. This ironically brings in Oblivion's problem with level scaling, namely that every fight feels the same.
So it's a skinner box full of references to better games. Does Elder Scrolls Online have anything to recommend it? Well yes. The skill system is good. It's got the Elder Scrolls level from use along with perks that can be bought using skill points at certain levels. There's plenty of skills as well allowing for a good deal of freedom that might not be apparent from the starting classes. Skills aren't all handed to you immediately either. They appear to go along with the guilds you join or character perks you earn. Werewolves and vampires are implemented in a manner that makes them feel earned. Crime is its own venture that can be quite profitable.
The game has undergone some huge revisions from launch. Tamriel Unlimited, One Tamriel with more on the way like Player Housing. It might be enough to keep your interest until Elder Scrolls VI. For me it's only fun with friends.