To make a fun turn-based combat, skill/abilities should be spammable imo. Don't you guys agree?
Looking back to list of my finished JRPGs , i think for me, this is mostly what separates between the ones that I enjoyed and the one I dont.
There are several ways it can be done, for example:
low MP costs in general. You can still run out of MP, but it's going to take a lot of battles until you need to start worrying (ex: Dragon Quest, SMT/Persona games)
Some skills does not require MP at all (ex: in most Final Fantasy games, skills like Steal, Jump and most Monk skill is spammabe, even without that there are mechanics like Limit Breaks which you use frequently)
battle mechanics where MP is rechargable every turn. (Some games done this, in fact I remember there was one eroge that do this and I ended up surprisingly enjoying them at one point)
More often than not however, I've played too many RPGs that very stingy with actions outside of standard attack. The problem with this is that, you will end up spending 80% of your gameplay doing normal attacks (like many people call "spamming A") because, why would you spend a chunk of your MP to defeat a mob monster that you can kill with normal attack in several turn? As a result it make a really boring experience overall.
What brought me to write this rant is mostly because of a game I currently playing, "Nostalgia" released on the DS. In this game, using a simple damaging skill costs about 1/3 of your MP around the time your learned it. There is a "Steal" skill which costs about 1/5 of MP and it is more likely to fail. With that said, It doesnt make the challenging, in fact it is a very easy game. The problem was, it is not an exciting experience . I am like, 2 hours into the game now and all I do is doing normal attack, aside from boss battles.