I'm not very far into the game (about 10 hours, I think) but I find the lack of a retreat option, changing of armor/weapons, choosing to defend instead of attack, and choose my party (I think that becomes an option later in the game, though) troubling. This is the first RPG where I feel pretty confident that whoever goes into battle is capable of getting the job done, though. Normally, there's somebody that I'd refrain from using because they're a big pussy. Not sure how I feel about the level-up system, either. But like I said, it's super deep and it works very well. I can't wait to play more of it!
*edit* after re-reading the OP, I have to say controlling only one pre-deciced person is definitely a negative in my book. I like to be in control of it all. And the level up system is actually more linear than something like the "Grid" from FFX. The Crystarium (at this point in the game) only allows me to build my characters in only one way. After that, even if I have extra Crystal Points, I can't build up my characters anymore unless we finish a particular story point. --just a couple of minor quibbles that I have. Like I said, it's just a matter of personal taste.
