Most important:
The ability to make the actor move during skills. - Not only the ability to move, but the ability to add pauses, have the actor hold poses, and define WHEN the damage is dealt. With the system in XAS, you can make the actor shoot forward, damaging all enemies in your path, then cause an explosion after x amount of squares quite easily.
The ability to define the rate at which the skill/weapon animates. - Although this is a tad tedious in XAS, you could make this feature available in bABS with just two configuration options. I think with this, you should also add the ability to hold the beginning and/or the end pose for x amount of frames. Doing this allows much nicer looking attack animations. Let's face it, the bABS animations for attacking and casting spells are kinda...meh
The ability to define a move route for skills. - This ability makes all the attacks in the game feel so much more original.
The ability for events on the map to wait for input from a certain skill or weapon. - this is the biggest reason I think anyone would choose XAS over bABS. Even with XAS' confusing configuration system, making a golden sun style mind read system, or a "press the switch in the right order" puzzle (think Zelda games) only takes a few minutes. To prevent lag, you could make "listener" events with \l. You could then set up a variable that we can check to see if its been hit by a certain weapon or skill. It'd be nice to have a "is hit by something" , "is hit by item", "is hit by spell", and "is hit by weapon" detection system.
Multiple attack combo (this is an addon the XAS community made) - This is a hard one to describe. What I am thinking of is the ability to define multiple sprites to be used in continuation of eachother in a combo. Think of games like tales of Symphonia, Kingdom Hearts, and Zelda (at least the 3D ones). If you just press the attack button once, they'll execute an attack, then have the brief attack penalty. But if, however, you press it multiple times, they'll atack multiple times THEN have the attack penalty. If you want it to be better than the XAS one, it'll need to support combo limits (aka, each actor can only preform x amount of attacks in a combo. This limit should also be changeable via script call), multiple sprite poses in the combo (when you attack only once, it'll use sprite pose 1. But if you do a combo, it'll go to sprite pose 2. Then after the penalty, if you attack again it'll start with sprite 1), the ability to define different ammounts of damage per each combo item (if you want the last attack in the combo to be stronger than the rest), and possibly the abilty to set up delay times for each item in the combo (wait 5 frames after attack 1, then 2 frames after atack 2.). Also, a nice ability you have with the XAS combo addon is you can define animation speed for each individual section of the combo (attack sprite one animates really fast, but attack sprite two goes slow, ect.) Sorry if this one sounds confusing, I'm having trouble describing it.
Kind of important:
The ability to hold down the attack button, which would charge a spell based on your current weapon - This would only take a few lines of code, and it could just be an addon. But it's one of those things that makes the XAS demo so flashy. I know you can do something similar with CRLS (or however you abbreviate it), but maybe you could create an addon that would make the CRLS system work in a similar manner.
The ability to have animated idle sprites - you have something sorta like it, but one feature that would be easily implemented, one that impressed quite a few XAS users, was that when you weren't moving or attacking, it would animate an "idle" sprite (usually just the actor breathing heavily) It'd only take a few lines of code, so why not?
The ability to create a spell that will 'seal' a certain action - It is like having the silence ability, but also for skills, items, weapons, blocking, ect. It'd be cool to be able to make a skill seal individual abilities, such as jumping. Maybe even add the ability to make seals that effect skills of a certain element (aka, 'seal of water' makes it so you can't cast fire based spells)
The ability to create a spell that will cast random states you predefine - not sure if you can do this in bABS, but in XAS you can make a skill that will cast random status ailments (out of a list you define). I think this would be a very useful feature, as you could make some cool spells with this. (aka, a song that targets everything on the field, and will either paralize the actor, or make them stronger)
The ability to call random battle cries - In XAS, you could easily make it so everytime you attacked, it would call a random battle cry our of a list of files for a certain actor (it required eventing or a script call). This is a simple feature, and adds greatly to the immersion level of a game.
Doesn't really matter
An on-map death counter - I've never really been impressed by death counter's, but it IS a feature of XAS. It'd take a line or two of code.
The ability to set a switch to true, rather than call gameover. - I know this would take one line of code, but not everyone knows how to do that one line of code
Non-pausing shockwave skill - Not a real biggie, but it'd be cool if you could have a shockwave skll that didn't pause and tell you who the attack would hit. Just preference on my part.
I'm not sure if you can do this yet in bABS, but the ability to have states that effect the movement speed