Just some thoughts. I'm kinda jumping around a bit here.
I've been playing Final Fantasy since the first one was released in North America. I consider FF8, FF10, and FF13 to be the low points in the series. Not including the online games, this means I dislike every second main Final Fantasy since Final Fantasy 7. So, going by that track record, I should love FF15. Haha.
This is due to a number of reasons, but one common theme for me is unlikable characters/story. FF10 and FF13 also share a much higher level of linearity too. But I'm really not into silly Japanese archetypes and situations. I almost turned the game off when Tidus and Yuna did that laughing bit. I couldn't stand Lightning, Snow, Hope, or Vanille. Oddly enough, the most likable character in FF13 for me is a guy who keeps a frigging chocobo in his frigging afro. Just think about that for a second.
I thought I would feel the same about Vaan based on the promotional artwork I saw in 2004, but he didn't turn out that way, and despite being the main character, he was only a minor player in the events happening throughout the game. And then you have an excellent cast of characters including Bosch, Balthier, and Fran. FF12 brought the serious back. It brought the adult back. Couple that with the ability to explore again, side quests galore, as well as great music, an actual FANTASY setting, and a different, but very enjoyable combat system (reminds me of a much more customizable Baldur's Gate or KOTOR battle system), and the game was a winner in my eyes. I look back on it very favourably. I plan on playing through it again soon.
FF13 took everything that was awesome about 12 and pissed on it. Exploration? Gone. Mature characters and writing? Gone ("moms are tough!"). You start the game not knowing what you're fighting for. And when I did find out, I didn't care. Nothing made me invested in what was happening. I just kept running towards that flashing goal on the map, fighting and running, fighting and running, wanting to be done with it. I hope Square has this out of their system and leaves this game and all its mistakes squarely in the past.
I am cautiously optimistic about FF15. The larger explorable world is a huge bonus. I just hope the characters, writing/lore, and battle system do it justice.