I've heard advice against over-questing lest you get over-leveled, and I think that's sound advice because I'm definitely starting to feel that way. I appear to be about five levels above what the game expects me to be, and I'm basically able to plow through any normal enemy or even any story boss. Obviously the level 70-90 monsters wandering around will still crush me, and most unique monsters are still a challenge even if I'm a couple levels above them, but yeah, maybe it's not the best thing to deviate from the main story for too too long.
You may have noticed that some quests have a clock icon next to them, though--those are the ones that eventually become missable once you progress the story too far. It appears the cutoff point (at least for the handful I'm on) is pretty obvious though, and it's a choice you have to approve, so you do see it coming.
On another note, I now have six party members (the pacing at which new members show up seems...off), and kind of don't know what to do with them all. With your "original" party you have a tank, a healer, and your main character (damage guy?), the first two of which seem pretty essential, so I'm not sure how to slot in the new guys. The sixth member seems particularly...odd. Any advice from someone who's played through the game already?