It's slow, cumbersome, barely interactive and just all around not fun.
Things that could be done to improve it:
1. Faster reaction times to improve immersion
The fact that I press the button and a second and a half later my character executes the attack completely disconnects me from the action and ruins immersion. I feel more like I'm a ghost floating over the character's shoulder telling them what to do than the actual character. When I push a button, I want something to HAPPEN, not for it to happen at some point later to be determined by lag, queued actions, etc.
2. More reasons to react to the situation
Mobs don't do many things that require reaction, the only exception being TP attacks, most of which have too short of a charge time for the player to react with an attack to interrupt the move (not that players have many attacks that can interrupt TP attacks, mind you). This means that the average "fight" is little more than spamming attacks and hoping the enemy runs out of HP first. With no real "twitch" gameplay in this game, combat needs to have some element of strategy to it and I mean BASIC fights with even the lowest level mobs need to have some element of strategy. We shouldn't have to fight NMs to experience interesting combat.
3. Fewer abilities that do more
I can't help but see many of the actions/abilities we acquire as simply not being worth the space on my action bar, especially when that space could go to a more useful action. I'd rather see half of the abilities culled out or rolled into the same attack.
4. More ACTIVE abilities!
We need fewer abilities that meddle with our stats or do something for our next attack and more abilities that DO SOMETHING when we push the button. Give us more stuns on command, knockback on command, abilities that influence the battle WHEN WE PUSH THE BUTTON instead of later on.
5. Why do we have stamina AND cooldowns?!
One or the other would be fine, but why do we have both? The whole point of stamina would be to limit the number of actions a player can take within a certain period of time. The whole point of cooldowns is to...limit the number of actions a player can take within a certain period of time. The battle system doesn't need both stamina AND cooldowns. My vote would be for removing the stamina bar as it adds nothing to the game, since there is no semblance of "stamina management", just mash the button silly to ensure the attacks happen as fast as possible.
If we're going to have a stamina gauge, it should do something to enhance the battle system, not simply limit the amount of stuff you can do, as cooldowns do that for us already.
For example, make it so stamina isn't used by normal attacks, but:
-Hold the attack button (of whatever attack it is) to allot more stamina to the attack. More stamina = more damage, but less chance to hit (like DnD's classic "power attack").
-Use an ability to turn stamina into MP in the middle of a fight.
-Ability that allows you to burn stamina to run at 2X speed until you run out of stamina.
-Ability that burns your stamina down but multiplies your evasion rate by 4X while doing so.
-Ability that doubles your magic potency and your casting speed while burning your stamina down.
-Ability that doubles your damage output while burning stamina.
All of these would make for very strategic choices the player would get to make, and more choices is what makes a game fun.
As it stands, I just can't see bothering to level to 50 in any class under the current battle system. It needs to do something to keep me awake through a fight and right now, it just doesn't.