Like: Always unique and fresh with each game, always trying to improve in different ways from the past releases. Beautiful Visuals, even though that's not what I look for in a game, and definately never the most important thing, I do appreciate the effort they put in. My favourites are 6, 9, 10 and 10-2, and 13 (and probably 13-2 when I get it) The reasons being, for 6 and 10-2, its the sidequest/story mechanices, the stories are touching and the way there are sidequests for each character in 6 really fleshes them out. It treats every character equally, and you really feel that. In 10-2, I like how you can influence how everything turns out in all the individual stories in the Final Chapter, and you can see them all develop and there are so many different results and little stories.
I like 9 for what I love most in video games, and that's character interaction and character development. Story and character are the two utmost things I look for in my RPG, and this game had it. I loved the little events in the towns and how it gave every character their due attention, cutscenes weren't always ZIDANE AND GARNET, you got to see what the characters were doing.
10 I liked for mostly story, it had lots of nice concepts.
13 had good characters and story, the actual mechanics were not as awesome, but the story was good enough I overlooked it.
Dislikes: A lot of the games the characters felt like cardboard, they had some character yeah and could keep interest, they just didn't interact ENOUGH or didn't make me FEEL like they were good characters. The best example for this was 12 for me. But a lot of them just didn't suck me in or make me feel for the characters.
I really do enjoy real-time so much more then turn-based, 12's attempt to combine them I kind of liked, but I also kind of never really got the hang of. 13 made that a bit better with its encounters, but I would have liked the ability to move independently to have been kept in battles. I'm sure whatever comes next in the main series will be even better.
A lot of the games may have had okay characters, but they needed more sidequests that involved characters and their dialogue together or comments on what's happening, rather than, DUNGEON, RETRIEVE AWESOME THING. Or some mechanic or events that had them interact just a little more then the mandatory story scenes, or obligatory because something has happened. Little things that compliment the whole a long the way go a long way. EX: FF5 has pretty good characters, and they actually interact a fair amount, not sure why its not a favourite really, but it was pretty good for giving attention to each and every character rather than some small focus on a party member just for their story segment, then they are just tag-a-longs, really.
Overall Direction: 12 lacked in story(well enough to keep me in it, but looking at it from far it was some good concepts) but the gameplay was a move in the right direction. 13 FINALLY brought in a story that could keep me in and good, fleshed out characters, but the mechanics were somewhat lacking. I think overall the series has done enough to make me remain a fan, especially with games like Dissidia that was an amazing spinoff that helped me appreciate the series as a whole.
For reference I've played: 3, 5, 6, 9, 10, 10-2, 12, and 13, and I know the story and characters (and have seen scenes and had friends play) 1, 2, 7, and 8. I actually fairly love the 7 series as a whole.