It is true, this is a rather grind-heavy game - it's not as extreme as a lot of Korean MMOs, to be fair, but it's still a lot of grinding. I feel like the experience is very improved by having friends to play with you; I have three other friends that are roughly my level, and we have an excellent team put together to tackle dungeons with. I don't mind the grind at all, though; this is one of the few games where I find it enjoyable, perhaps because every time I pick a new fight, I have to put a little thought into my initial placement and which skills I pick to speed up the process most efficiently, and it can come down to a management process of "should I grind on this big clump of enemies and risk the chance of dying, or at least having to eat a shitload of food to make up for my lost HP and get back to grinding, or should I take them on one by one and exploit my knowledge of this particular enemy's amount of movement points?"
And the dungeons themselves make a fun little grind, with their starting positions and environments smartly designed for you and your friends to set your placement; it's a "tactical grind," I guess you could say. It's not exactly the same as "hold the arrow key until you run into a random encounter and then slam the attack button until you level up for the next two hours," the kind of grinding I abhor.
There are a couple of balance issues, I think, between the classes and especially equipment sets that need to be worked on, but Ankama has shown that they actually care about game balance in the past. I was hooked on first play when I got this - I really couldn't say no to turn-based isometric combat if I wanted to.
The problem of "you're rewarded for playing fast" is resolved by having friends to play the harder encounters with. You're all discussing exactly what you're planning on doing the entire time (hopefully), so when it comes down to your turn, you already know what you are going to do. At least, with my friends, if you're playing slow, you get yelled at!
There are a few classes where it's better to max AP or MP (Air Sacriers are all about MP), but with, say, a fire Feca's Shield, like I'm building, it's really not practical to put 30 levels worth of ability points into boosting your AP, because Intelligence is much more valuable. Even if I build up to get the absolute maximum number of AP-per-turn, with item sets and point allocations after all the leveling, I can only get off maybe 1 high-power attack and another moderate-damage spell in one turn... And they're severely crippled by the fact that I have no Intelligence, so there's no significant damage bonus on them.
While it is technically true that you can max every spell in the game, you won't be using 2/3 of them nearly as often as you're using the ones you specifically specced into. You will have picked your equipment and ability distribution to make your particular choice of spell element most powerful, and using any of the others is a waste of AP; and, well, it takes a fucking lot of time to level up a spell all the way to 100, and it's just not practical to try to train every single possible spell choice.
I don't by any means think it's perfect, but me and my friends are having more fun with this than any MMO we've ever played together in the past.