Eight things which I consider bringing me the most fun when playing a MMO:

1. Challenging end game content with lots of variation, both instanced and open world.

2. Character devolopment up to(leveling multiple jobs/classes) and above the level cap (merit system or similar).

3. Both large(8-24) and small(2-6) scale party content like open world dungeons, instanced battle zones, campaign/behest large scale battlefields, BCNM type battles.

4. Regular as well as rare world events (monster invasions etc).

5. Many long mission based story arcs to play trough, with challenging boss fights and cool cutscenes.

6. Exploration of varied and exotic lands.

7. Rare monster hunting like NM and HNMs.

8. Longer Quest chains which affect the gameworld (least trough your eyes with a form of phasing system or similar) with a good story to tell.
Like saving a small town from a threat, aid the rebuiling of a outpost, unmasking a high official NPC as a traitor etc.