While Lufia 2 is a very good game, it has a noticeable flaw. Namely, it is extremely linear in one of the worst possible ways, and it tends to call attention to that fact.
For at least the first half of the game, Lufia 2 has a very predictable structure. The world and plot are broken up into distinct chunks and islands, each of which is self-contained. Each region has a single town, a single dungeon, and some kind of barrier preventing you from traveling further along your path. Each town has some kind of problem that needs to be solved, and after you go to the local dungeon and solve the problem the path leading to the next region opens up, where the cycle repeats. There is very little variation in this structure. For the most part, there isn't even much variety in the kinds of barriers that block your way. Five of the first nine regions use "warp shrines" that happen to be either closed down or malfunctioning as the barrier. Two more are towers that happen to have locked doors, and the last two are caves. By the time you go through the fifth warp shrine and move on to the point where side-quests become available, the entire first section of the game has been completed and Maxim and Selan have already been married.
The basic formula being used in Lufia 2 is hardly a bad one. After all, it is the dominant structure of RPGs, and it is used so much because it works. Dividing up the game into smaller chunks based on geographical region keeps things simple and organized, and lets the player focus on a single challenge at a time without a lot of distraction. It is a good way to keep introducing new places and characters without overwhelming the player or letting the player accidentally wander into overly challenging areas without warning. That said, Lufia 2's problem is that it sticks to the structure far too rigidly, and uses unreasonably artificial means in order to enforce the structure. In the early stretch of the game there are no sidequests, no plots that involve more than one town or dungeon in order to resolve, and the whole structure becomes predictable and repetitive very quickly.
If there has been a few more places in Lufia 2 where you could go on a sidequest or simply explore a bit, the structure would not be so confining. For example, they could have made an optional dungeon available from early in the game where you could go and try to earn a Capsule Monster (a type of ally monster that tend to be overly rare in the beginning of Lufia 2). Perhaps they could have used a boat crossing as a barrier rather than a warp shrine, and given the player a chance to go to an optional area like the Ancient Dungeon earlier in the game. Anything is better than using the same trick over and over.
Game should have variety and should provide the player with the ability to actually explore. Games should not stick too much to a single structure, even if it is a good one.