Radiant Historia

It's basically impossible to miss anything in Radiant Historia, any completionism is relaxed and can be done at your own pace and the system's pretty self-explanatory. Half the fun is bumbling through it, seeing where you end up, then going and searching old locations again - you will need to do this to some extent, as there's a kind of Zelda-ish progression of dungeon tools available for finding new stuff when you backtrack.

Aht is the strongest character in the game. She gets traps that can do a ton of damage to enemies on the field, and her sidequested traps are twice as strong. Stocke, Aht, and Gafka is the dream team. Gafka has a skill that can move every enemy on the field to the center, which allows Stocke to knock them all in to Aht's traps as many times as you can manage.

Pre-emptive strikes are absolutely necessary. Some asshole on the dev team decided that every enemy in the game has an obnoxious speed stat that will almost always be higher than your characters, usually giving them multiple turns before you get to go.

Take time to level up new characters, especially Aht, as they tend to outstrip your starting characters at higher levels. When you unlock various powers you'll want to backtrack for sidequests and new chests to get, but otherwise you can pretty much go forward however you wish.

There is one sidequest, At Journey's End, that has obscure/annoying to get requirements. You CAN go back and get the requirements, but they are nowhere near a node so it is a lot of annoying backtracking, so I would recommend reading up on a FAQ to get the requirements as they come up. If you want to get all the sidequests without referring to an FAQ, just talk to everyone you can. Make sure you talk to them again if the chapter changes, as they will have different stuff to say/sidequests maybe.

There are some nodes (such as the first one where you get the big branch off) that will have a seemingly mysterious path. It will be filled as you play the game. Don't worry about it.