Dungeons & Dragons Online is the 2nd MMO I played, and one of the early MMO's that brought the F2P, microtransaction model of business to the Western scene. The game was based on, you guessed it, the classic tabletop game D&D using the 3.5 edition ruleset.
DDO differed from other MMO's in some very drastic ways. First, party roles were not strictly defined, and you can even take levels from different classes. So, it was possible to build e.g. a Fighter who can cast heals, or a Cleric good at melee. Instead of requiring the "holy trinity" or tank - healer - dps, roles were defined by what kind of challenges the content had. If there were many traps, you'd recruit a Rogue who can disarm them. If there was an underwater segment or a puzzle with a high STR check, you'd look for a Fighter with a lot of STR, etc.
Secondly, the core gameplay loop in DDO is not endgame raiding (though those were present), but rather reincarnating your character! Once you hit max level (lv20), you can buy items from the cash shop that will reincarnate your character at level 1 with a new and permanent "Past Life" buff. The same past life buff could stack up to 3 times, different jobs gave different buffs, and you can keep reincarnating your character to get more and more buffs. Just imagine if, e.g. in FFXIV reaching Lv80 with a PLD allowed you to reincarnate at level 1 with a trait that gives 2% damage reduction, doing the same with DRK gives 3% magic reduction, WAR gives 1% lifesteal and GNB gives 1% dps... an endgame raider who plays tank would be forced to level all 4 tank jobs 3 times each, just to perform their role optimally.
Thirdly, in this game, spell points (SP) did not regenerate - not even the healer's SP!
HP/SP is only recovered at a rest shrine, which are placed by the dungeon's designers. This is to model the D&D style of Vancian spellcasting, where casters must prepare spells in advance and only recover spell slots at the end of the day's rest. In fact, even non-mages frequently had such limited abilities as well. For example, my character was an elven Paladin, and I got like 5 uses of Lay on Hands (a powerful heal, like Clemency) per day, which regenerated at a rest shrine.
As a result, DDO was a game of resource management and organization - making sure that your party has the right skills and traits to clear the dungeon, and rationing your spell points and resources so you don't run out. Experienced players knew exactly how many casts of Fireball were needed to delete the group of trash in this or that room, and made use of wands, scrolls, potions, and magical items to supplement their resources.
This is also the game that taught me that the role of the healer is not to just heal, and that at times, the healer's dps and other powers are more important than their healing. After all, there is no point for a Cleric to spend 100-200 sp to cast Heal on the party members who are fighting an enemy, when they can instead cast Finger of Death for 50 sp and just delete the enemy. In fact, the Cleric who does the former carelessly may very well run out of SP.
By the way, some powerful enemies can cast such Death spells too! A battle between Wizards and Beholders would be like a sniper duel, both sides trying to 1-hit-kill the other first. I think this is the only MMO in all history where trash and players can both one-shot each other.
But the game became imbalanced around the time I played. In particular, items called "clickies" that gave charges of spells became prevalent, and they gave all kinds of powerful buffs like Death Ward, Displacement (50% miss chance!!) and Freedom of Movement. There were even items that restored SP. Just imagine that you are playing BLM, and there is an item from Thordan Ex that gives you 3 charges of Requiescat per pull (+100% magic damage), another item from Hades EX that gives 5 charges of Pesence of Mind per pull (+30% cast speed), and finally an item from E4S that gives permanent Arms Length (KB immunity). Now imagine someone who really can farm all such items, and you have a good idea how ridiculous optimized DDO builds can be!
The prevalence of such cross-class buffs, and the tendency of the playerbase to favor characters who were not dependent on healers, gave rise to "BYOH" builds ('bring your own heals'). While it originally meant characters which did not need a healer, later on it became a shorthand for self-sufficient characters who can solo most of the content. People would often advertise parties as "BYOH" to indicate that there is no healer and you are expected to take care of yourself, including knowing the layouts and puzzles of the dungeon.
Eventually, as the trend grew towards its logical conclusion, people started just making soloist builds that did not need a party, and the rationale for playing with others faded away.
I only played DDO a short while, probably around 6 months or so spread over a year or two. But it's interesting how much influence this game had on my attitudes towards MMO's. It's because of DDO that I feel an MMO should not be solo friendly, that healer should focus dps, and that f2p games are bad. But I didn't want to level my job up again from level 1, and I had no real friends or attachments there. Soon after I reached level 20, I lost interest in the game and left.