Oh man, here I go. You’ve been warned. 
Guild Wars is unbelievably, wonderfully unlike any other MMO in regards to the endgame.
For starters, levels 1-80 are really more tutorial levels than anything else. A way to get comfortable with character mechanics before you get to the “end game”.
Here’s the thing though - for a lot of MMOs, the “end game” is dungeons to get better loot and armour, Guild wars is not. With a few exceptions, the armour you get at level 80 is some of the most powerful you will wear. Dungeons can lead to better looking armour, but not better stats! Most people, nowadays, don’t run dungeons as their endgame!
There’s a few choices for what “endgame” means.
- First and foremost, there’s your personal story - a story crafted by you, that ends with an encounter with one of Tyria’s five elder dragons, Zhaitan. This is soloable all the way to the end.
- Once this is complete, there’s the living world. Season 1 wasn’t designed to be replayable unfortunately, but Seasons 2 and 3 are, and they progress your character, and the story, forward, complete with unique weapons and armour and lead into:
- The expansion Heart of Thorns - this opens up a whole new half of Tyria, The Mastery system (gives all your characters special abilities, like gliding, after 80), massive new dynamic content, Including, if you so wish:
- 10 man raids. The raids do not provide more powerful loot, unlike almost every other MMO! It provides unique looking armour, so if you’re interested, you can fight there, but there is no requirement to do so to max your stats/damage/whatever.
- If you’re not into that, there’s also the Fractals of the Mists, which are small mini-dungeons designed to be played by one group of five in about 10 minutes each. This is Guild Wars 2’s progession mode - you work your way up to harder and harder versions of the same mini dungeons for, again, better looking, not behaving, armour and weapons
- If none of that interests you, there’s World vs World - described before as being a soldier in an RTS game: huge 100 person fights over towers and keeps with siege equipment, gates, and giant golems that runs 24x7x365, with your world pitted against other player worlds, and
- Lastly, Structured PvP, 5-on-5 conquest player vs player combat with normalized stats
Guild Wars 2 isn’t raising the level cap beyond 80, they aren’t making you go to dungeons (or raids, or fractals) to get the best performance from your character, and Living World content is designed to be done either soloable, or in a group, your choice. Basically, you do what you want.
AFAIAA, there’s no other MMO out there where you can choose from such a wide array of tasks at the “endgame”, where you can do as much casual solo or grouped as you want and still end up with gear as powerful as hardcore raiders have. The entire world is viable because you level-adjust to the content there, but still get loot appropriate to your level, (so you can always play with your friends), and the game has been carefully designed so you never end up in the world fighting another player for resource nodes, kills, spawns, quest items, whatever (PvP areas aside, of course).
It’s not perfect, of course, but if any of those endgame modes motivate you, then it’s a heck of a lot of fun. 
