I like JRPGs. It's one of my many vices. And at some point, I'd like to make one. Because of that, I've spent some time thinking about what should be kept in mind when designing a JRPG. I doubt anything I've come up with is revolutionary or original, but I'd like to run through some of it in the hopes of collecting feedback. Remember that this is all just my view. If you think I'm wrong, please, tell me why.
Anyway, there is something that I think is the most important principle to keep in mind when designing games, JRPGs in particular - and that is the Principle of Jesus Christ How Did They Figure That Out, which I'll henceforth refer to as "Players are smart".
What I mean by that is that if there is an optimal strategy to your game, players will home in on it like... well, a homing missile. If your game is any good - and probably even if it isn't - within a few months the gaming community will know practically everything about how your game works. Hell, they'll probably know more than you do when they discover that weird bug you can use to completely break the game. And there is absolutely nothing you can do to stop players figuring out what the game mechanics and optimal strategies are, short of not releasing the game - which seems somewhat pointless.
This is important because of the vast corollaries that follow from it, but also because it teaches you to be wary - program defensively, because any weird glitches you leave will get exploited. Try to design the game such that there is no optimal strategies - there should be a number of effective strategies that players will employ. Otherwise, you'll find that everyone will end up doing the same thing. Consider, for example, Final Fantasy 8 - basically, all you need to do is damage the main character (Squall) down to very low HP. He can then, every round, use a special attack called 'Renzokuken', that does a pretty good amount of damage - and by the end of the game, has a chance of activitating another special attack called 'Lion Heart', that does very nearly 170,000 points of damage to pretty much everything in the game. That will one-shot pretty much any boss. It didn't take long for players to catch onto that, and the result is that FF8 has a pretty boring battle system that has to be spiced up by artificial limitations - playing the game without using any limit breaks and at the lowest level possible, for example.
Unfortunately, testers are not good at discovering the seriously broken combinations of effects, unless they're blindingly obvious. You just don't have enough of a population for long enough to figure it out. That means you're going to have to put some thought into the game and try to work out what will break. Try to play it like your players will - like it's a puzzle that needs solving. At the very least, consider what happens when the same effect is stacked - what happens if the player does everything possible to increase his crit chance/evasion chance/counter rate/attack/etc. In the process, assume that the player knows and understands every single formula and game mechanic perfectly and never makes a mistake. The hallmark of a good JRPG is that it will still hold up under those conditions - the player will win, yes, but with some level of difficulty, thought, and, most importantly - there will be several different ways they can do it