It has been a while since I've updated my review. Last time it was in 2016. Lemme make a new one!

Dungeons & Dragons Online (DDO) is an MMORPG where are both explorable areas and different dungeons and raids you can do with your character, which depending how you build them. The game also has different difficulties you can run dungeons with, as well as even harder difficulties once you get to the late game. The game heavily leans toward co-op playing with either your friends, guildies or strangers.

I have played this game far longer than the Steam says. Currently I'd estimate it to be closer to 6500 hours now. Here are my takes on the game:

+++ Character creation and builds. Easily the best thing about this game. You have so many options to choose from how you want to play this game that it's fantastic. There are 15 races (of which 4 are premium, excludes iconic characters) and 15 classes (of which 3 are premium, too), as well as nine archetypes of those classes. Not only that, you can multiclass as you want, giving endless amount of options!
++ Narration and atmosphere is perhaps the most unique aspect of this game. With the DM narrating what you're doing, and scenery changing a lot depending on the quest, the game definitely gives you the right feel that you are in a real world.
+ Reincarnation system is a way for level 20+ characters to basically start from level 1 again with certain benefits. It's a fantastic way for you to try out different classes, races and builds while also being able to enjoy some of the quests you might've missed during your "first life." This is also the way you can access to harder difficulties right off the bat without repeating quests and without paying for VIP.
+ Difficulty settings in dungeons for both the starters and hardcore players.
+ Co-op heavily encouraged. Gather friends and you will have fun.
+ Large explorable areas and lots of different dungeons for you to try out playing.
+ Community Is there for you to help for any questions you might have, let it be in-game or in Discord.

-- Complexity. The game can be complex to new players, especially those who haven't even touched the original DnD board game. It doesn't help that the game doesn't a really good tutorial or pre-made builds. You have a lot to learn, so DDOWiki will be your best friend at guiding you through the mechanics, like spell saves, multiclassing, favor, and so on. I do have made a introductory video for people to alleviate this, but even then, the knowledge is like a black hole.
-- Paywall. The biggest issue of the game by far once you've gotten accustomed to the game. A lot of the content is behind needing to purchase with real money. Sure, you can earn premium currency as you just play but you need content for that. HOWEVER, if you manage to snag a certain code during promotional periods, you might get most of the content free. It is almost necessary to snatch one of these up.
-- Lag and glitches happen a lot. They're not usually game-breakingly bad, but they are very noticeable when they happen.
- Small playerbase. Each server is rather small, with around 300 people per server (except Wayfinder which is a dead server) during peak US times. You are most likely playing alone unless you find a guild to play with, find people to play with, or with a good chance, someone joins you in grouping tabs.
- No PvP. Sorry, but there is little to no way to fight against each other. This game is more focused on group play.
- Graphics are out-dated. This game is from 2006, and they haven't really still updated the look of the game in old contents. Expect floating objects and wonky graphics.
- Little to no endgame. Because of the above said reincarnation system, most people opt for reincarnating instead of staying at the maximum level (which is 34 atm) and doing raids. Though, that's really is your endgame: Raiding. And you need the content bought for raiding.
- Steam Client Can be a little broken. If something is not working for you, try the standalone client from the DDO website.

This game is what I'd call a diamond in the rough. It has its issues: the bugs, the paywall, the lag and the rough learning curve. However, if I personally look past all that, I can say you have a fantastic game beneath it all, a game that is very enjoyable with a group of friends.
It is just a shame the paywall is such a big issue that it is hard to recommend the game without the code that unlocks most of the content for you - or if you're willing to drop the minimum of 30 bucks for the most crucial adventure packs to make the leveling at least somewhat comfortable. So I'm hoping that one day, they're just making all the code's content forever free. Here's hoping for that future.
I am recommending it however. At least, give the game a try. Maybe wait until the code is available once again - or when adventure packs become free, who knows.

I also recommend looking at Josh Strife Hayes' video about this game if you want a deep look at it from a new player's first impression.