Yeah I think experience with FL is going to be important in that after a few hundred games you develop a sort of sense for when you are drawing attention, when you need to move back, when you can play aggressive, etc. Like, for example, weaker players often panic when tanks are on them, and feel the need to attack them. However, a more experienced player will know that tanks are not inherently threatening and can be ignored, and the majority of their danger only comes if their CC can put you in jeopardy.

Spell casts are such that you are, as you've noted, stationary while doing them, but the payoff is also delayed. This means prediction is huge, and being able to read a lot of data quickly in advance is critical. You have to know when an enemy alliance is being distracted by your tanks and melee. You have to when an enemy formation is falling apart and you can start to move forward. You have to when your presence here will be enough to force them to retreat.

No matter what role you play, you have to be able to check your surroundings and map frequently to get an idea of when your teammates have your back or when you are overstaying your welcome. Casters and healers are privileged in that we can check the map or swerve our camera around while we cast; during a 2 sec period of a spell cast, you can be gathering information or changing to a new target. Just like any other role, you will have to know when you are subject to attack, and since you can't zone with as much fluidity as a ranged physical you'll have to combine slide casting with instants to reposition.

I'd say casters (especially RDM) and non-NIN melee are harder to play well than tanks, ranged physical, and healers, so if you find your general game sense in FL is lacking, it can be worth it to play something more forgiving until you get more games under your belt. I've noticed a lot of the people who consistently play mages and melee well are people who have played a lot of FL, where as I feel if our best players are, say, a tank or healer, their talents are not being utilized as well.

Also, there's plenty of situations to get value on AOE, as there's quite a bit of geometry to work with. The centre of Onsai Hakair is obvious, but other key features are:
  • Bridges
  • The easiest point of reaching elevation
  • Corners
  • Near nodes
  • Any time a team pursues another, as people will inherently take the path that will most quickly get them in range of a target
I would say on average my AOEs get at least 3 targets. Of course, with good geometry, much more, though that should be the minimum you are looking at. A well-placed Painflare on 5+ players can realistically be the deciding factor for a node, and there is a huge component of patience in knowing when the time is right.