To me, a Final Fantasy honestly needs one underlying thing for me to appreciate it: charm. If there's no heart in it, it bugs me. Maybe that's ironic, since many people believe FFXII is the least heart-felt game in the series, but to me, there is plenty.
I just beat FFTactics the other day, and I think it should definitely have had a roman numeral to call its own. Yes, it was different, but it was superior in its own right. In any case, I've enjoyed all of the main series and most of the spinoffs of the franchise. The only day I'll ever call a main FF game not worthy of being an FF game is if for one, it lacks charm and soul in the story, and two, it's not an RPG of some sort. None of them have caused me doubt yet, so I'm not too worried myself. Square, in my opinion, just needs to go rediscover its roots and make a more classic-themed game, however, to regain popularity amongst the more critical fans.