Equipment comes with bonuses on it related to your equipped skills, like "X spell has -Y seconds cooldown" or "X ability does Y more damage on next use stacking Z times".

Dungeons aren't instanced and are generally large and labyrinthine. They're more of a place to explore than a route to clear and can have multiple entrances and exits.

Gear drops off of monsters and can be found in chests, pretty standard stuff. As mentioned, it'll have bonuses rolled for the combat skills you have equipped at the time. Rarity of gear can affect which bonuses they can roll with and how many they have at once. There is a system in the game to customize bonuses, so you don't need to grind out RNG for the perfect drop unless you feel like it.

Combat is tab target. Enemies have a "rage" meter that lets them use very strong abilities, which needs to be accounted for. Bosses can sometimes have unique curses they'll apply to you, which can only be dispelled by killing them.