It would be a good idea for us to take note of design mistakes that other RPGs have made so that we do not repeat them here. I'll start us off with one that has always annoyed the hell out of me.
1. Battles where the player is supposed to lose
These can be done right, but often times they are not. What you want to do is to have it play out as a normal battle that finishes at a point -before- all the enemies are defeated. If the only way for the battle to finish is for the enemy party to be defeated (which is impossible since the enemy party is given infinite health) or for the player's party to be defeated (all allies have to 0HP), that's annoying. Its annoying because the player doesn't know that they are supposed to lose and think they are just fighting some insanely strong boss character. They end up wasting a lot of items and mana/MP trying desperately to win an un-winnable battle. Then when they think that its game over, instead the game continues and they realize that they just wasted a lot of effort and inventory and got stressed over nothing. It drives me insane when this happens to me in RPGs

Like I said, the way to do it is to have it play as a normal battle. Maybe the end of the battle happens when you reduce the boss'es HP to below 20% of its max or something. After achieving the end criteria, the game takes over and the player's party is defeated, or a cut scene occurs outside of battle, without allowing the player a chance to retaliate. If the player's party is defeated before the end criteria is met, then the game really does end and the player has to try again. This way the player's effort is not in vain nor do those resources used in battle become wasted, since the player has to fight to achieve the end criteria. It makes so much sense to do it this way, I don't know why game designers insist on doing it the wrong way.
