Forewarning: Many negative reviews come from ESO's launch which was poor. However, Zenimax completely saved the game and made it something truly incredible since then.
Second forewarning: If you come into ESO expecting FFXIV or WoW (I've played both for a long while) leave. ESO is not those games and should not and quite literally cannot be compared to those games.
ESO is a pretty unique MMORPG. I've spent the last five years playing it relatively actively and I still find countless reasons to love it.
To start, ESO puts you in as fully voice acted, beautiful world that you are open to explore right from the beginning due to level scaling. All of ESOs lore is confirmed Canon which makes it that much more interesting setting foot in a part of Tamriel's history before Morrowind, Oblivion, or Skyrim.
The game allows you complete customizations of your character's appearance, build etc.. From class skills, to guild skills, to being a werewolf or vampire, racial passives etc.. Every character is essentially unique to you.
Though ESO does have a meta like most MMORPGs, you don't have to chase it to have fun. Thousands of in depth quests await you, friendly guilds for RP, raiding and PvP will help you, and most importantly all that matters is that YOU enjoy what you're doing.
ESO offers very in depth crafting (though fairly simple to do) you can do a lot with it. You can also make quite a bit of money off of it by selling it through guild traders as there is a player economy.
PvE is perfected in ESO with both public dungeons and traditional dungeons. Not to mention it still is an Elder Scrolls game so you have to expect them to give you a solid PvE experience anyways.
PvP is probably the most unique of any MMORPG. Take part in a full war in Cyrodil where you can use trebuchets, ballistas, build your walls, siege keeps and fight against hundreds of players. Or take part in Battlegrounds in a series of fun game modes such as team deathmatch or capture the flag.
^ Don't forget the daily rewards for doing both PvP and PvE content!
A sub isn't really required unless you want to be more involved in crafting. But even then it's still worth the price considering it gives you access to all DLCs, the craft bag, an experience boost and crowns to buy neat stuff with. (THE GAME IS NOT P2W) (All purchases are optional and don't serve any advantage over other players as players can still achieve the same things without money)
The developers (Zenimax Online) really care for and love this game. They've made beautiful expansions and though there are occasional hiccups, it doesn't appear they are giving up on anyone, or anything.
It's a fun game. It takes some commitment and it will eventually grab you. Don't run in and level to 15 and quit because "It's not fun" it takes some time to get over the first humps and once you level up further you'll see all the things ESO can offer.
Go in with an open mind. Don't expect Skyrim, don't expect WoW or FFXIV. Expect ESO. Have fun!