In that last mission, we met one of the various important NPCs, Lion. She says she is an adventurer just like you. If this were FF2, she would have been a guest character in your party. (In fact, in current retail, you can recruit a bunch of the important NPCs to be in your party in lieu of other players so you can play solo).
That fight was an instance battle, capped at level 25. However, the zone that would probably be a "dungeon" instanced group content in other games, in FF11, almost the whole world is open. So a higher level player can get you TO the boss fight.
Speaking of getting around the world, Vana'diel is BIG. Any kind of transportation you have to unlock, so you're walking to most places at least once. Chocobos have to be unlocked at level 20 in Jeuno, which means walking through two very dangerous zones. Hostile mobs will detect using sight, sound, or magic. So, if you can get access to Sneak and Invisible, you can safely walk most places. WHM can learn those spells and cast them for free at a high enough level. Ninja (NIN) can cast them using a ninja tool, which is a consumable, crafted by players. Or anyone can use a Silent Oil and Prisim Powder, which are consumables that are crafted by players. So, if you're a broke BLM (spell scrolls cost gil and get expensive fast), you can try to walk by carefully weaving between patrols.
Another important mode of travel is the teleport points, Dem, Mea, and Holla. WHM gets the teleport spells. The scrolls can each be obtained through a quest that is non-repeatable, but the scrolls can be sold. So many people would just buy them. many people looking for a teleport would offer to pay for a teleport and WHMs sometimes felt it was owed because they had to buy the spell. Which is what caused me to decided to switch my main to WHM and never demand payment for teleports.
The last way to get around is via Airship. They go from Jeuno to each of the other cities on a real time route. You can't get an airship pass until you finish this next mission though (or pay a big hunk of gil). The main advantage of airship travel is you can safely go do something else while you wait for the airship ride to finish.
Anyways, after a bit more effort with your home nation, your head of state will send you to Jeuno. You meet with the archduke Kam'lanaut and his (smaller) brother Eald'narche. The beastmen are trying to revive the Shadow Lord using magicite. You get sent to speak to Aldo, local contact for the Tenshodo. Aldo was the kid in the intro movie. And you meet his sister Verena. Basically, there's some goblins (another beastmen tribe) that live in Jeuno and trade with people and Verena is friends with one, but tensions are rising around the whole shadow lord thing. Verena is pretty much the only one trying to encourage peace.
You see a vision of a band from the various cities on an expedition to Xarcabard in the northlands. They find nothing and two of the party fall into a crevace and can't be found, Cornellia and Raogrimm. We learn that their companion Ulrich turned on Raogrimm and tried to kill him, but Cornellia stepped in front of the blade and was killed.
You venture into the beastmen strongholds and get the 3 magicite, but see a vision of the shadow lord. he claims that "anger, cowardice, envy, arrogance, and apathy will destroy Vana'diel and that we awakened him. We had the magicite over to the archduke and learn there's a 4th in Castle Zvahl (the shadow lord's home base in Xarcabard) and that the Castle is sealed and there's 3 keys to the seal, one key held by each of the 3 nations. Unfortunately, while we were getting magicite, all 3 heads of state were attacked and the beastmen have the keys to open the seal. also Eald wants to talk to Verana for some reason.
Next we learn that there's actually a 4th seal in Fei'Yin (ancient home of the lost race of the Zilart). There we meet Lion and a galka Zeid. He tells us that the vision of the adventuring party with Raogrimm we saw earlier was 30 years ago. Zeid tells us the 4th seal is broken and a new one will be useless. Your next mission will be to Castle Z itself and stop the resurection of the Shadow Lord. Of course you're too late. You learn Zeid is the one who cut down the shadow lord 20 years ago. And that Raogrimm is the shadow lord, awakened by his Galka rage 30 years ago. You fight and defeat the shadow lord.
And so the shadow lord was defeated thanks to your selfless courage and sacrifice
Through the clouds shines a symbol of newfound hope for the peoples of Vana'diel
But none should forget that wherever there is light, there is always shadow
There's more stories in the expansions, but that's the gist of the main game. Getting to many of those fights requires being high level and dealing with enemies that can not be snuck past. There's a ton more to the game and a lot more jobs to unlock. In my day, I was obsessed with unlocking Summoner (SMN), but to get the summons you had to do a level 75 fight which took having several high level friends, or doing a level 30 mini-fight solo. I tried those solo fights and I would get so nervous that my hands would shake. There's no other way to say it than FFXI was a tough game if you didn't have a good group of friends that are skilled players with enough gil to afford quality gear. I was able to connect with TheSL here at TT and I have him and Laharl to thank for getting to experience most of the story of FF11. We did a bunch of it after the level cap was raised, so the difficulty was lower, but not completely gone.
I have tried WoW and FF14. I do like 14 quite a bit, but I never got into any other MMO like I did 11. It was at the end of an era. WoW came out not that long after FF11 and MMOs got faster paced and offered more convenience. The end game became where the real game starts and so the leveling process became all about speed. MMO players became the locust swarm, devouring content ever faster. Guides and out of game resources go up instantly so everyone knows all the mechanics for every dungeon and nobody even bothers to explain it to you. There's programs that can log your DPS and perfectly min-max your rotations.
In terms of its spot in the lineage of FF games. 11 made me feel the way i felt playing FF1 when i was a kid. 11 is full of characters and political entities like in FF2. 11 entices you with jobs you recognize from FF3 and FF5. 11 can absolutely awe you with its sense of scale and unique zones that remind me of FF6 and FF7. 11 is full of unique races and impressive architecture like FF9. And I think 11 shares a slow pace with FF10 and and open sense of adventure with 10-2. Most similar though is the next game FF12. The elevator pitch i remember was FF12 is like a single player FF11. I bought FF12 at midnight of release day. And I've never finished it...