I think one of the most important things is to be observant. It can be very easy to just walk mindlessly through rooms and smack things as they walk up to you, but if you really want to learn, then watch and pay attention as much as possible.
Watch how enemies react to your positionings and your various actions, and learn their ai patterns (How many people know what causes bringers to charge their laser? What causes sinows to flinch and uncloak? The exact size/angle of delsabers' jump attack hitbox? Which player gibbles will target? What kinds of attacks you should to do stay above ill gill hp triggers that change their ai, while still being able to finish them off after? etc.). Watch how many attacks it will take you to kill enemies. Watch how you control their animations with different timings and different attack combos. Watch how you have to move to dodge attacks. Watch how spawns are laid out, where enemies start, when they appear, what triggers cause them, etc. Watch your own character to see how your attacks come out, when you can start queueing up more actions, when you are free to start moving again after not only different weapons, but also different levels of combo attack with them, how long it takes to get to running speed, at what point in your attack does your character step forward, how much actual reach do the attacks have and what the targeting angles are like, how long you can delay your combos in between attacks, how cancelling attacks or dropping targeting affects these things. Watch how your teammates move and react to things in the room (remember, you don't always see the same things as them, which means knowing how monster ai works is very important to understanding your teammates' current situation). Even little things can make a difference like watching how unlocking doors act based on your position (being pressed up against a door keeps you stuck against a wall longer; it's significantly slower than running into it as it unlocks which not only allows you to start at higher speed, but you also get through the invisible wall itself much faster).
PSO has a very heavy focus on action more than rpg, and it's the many small details that really inform your approach and allow you to play skillfully (challenge mode is especially good for teaching many of these details to players, and is a good test for separating out the scrubs from the experts). It's things like good timing, precise movement, understanding enemies, understanding yourself, and all the other nuances to the game that are required for high level play.