1. Fantastic exploration - FF has the unique attribute of using a new world every game, meaning every game is a journey into the unknown, something all games should strive for. Even the simplistic story of the original Final Fantasy is full of unique fantasy elements.

2. Recurring themes - Cid, Chocobos, airships, Black Mages, Moogles, Crystals, the big bad evil guys, and so on, had been in my head since before I even played a Final Fantasy. It's great to see some familiar things appear, and it's great to see them evolve over time.

3. Difficult, strategic battles - Sure, this can be avoided with grinding, but without maniac grinding, the old classic boss battles would rely on tactics and preparation. You weren't coming out on top if you had no idea what you were doing.

4. Music - The FF series has some of my favorite video game music.

5. Nostalgia - This ties in with #2, and speaks for itself. Chocobos conjure up memories of playing Chocobo's Dungeon as a kid(my first "FF" game), the pointy BLM hat reminds me of the hours spent in FFXI, and so on and so on...