Combat is very different from other MMOs. There is no auto attack; every action is a response to a command from the player. Attacks can be avoided by positioning, and it is actually required for success to avoid attacks as much as possible (or in the case of the GF, to block attacks). Powerful enemy attacks have a red warning which varies but the most common are a circle around the mob, or an arc in front of the mob, or a straight line/rectangle in front, and the like. Players are expected to get out of these targeted areas to avoid the big hits. The necessity of dodging attacks makes combat very active and pvp-ish, as the player and enemy are almost never just standing still beating on each other (as in many classic MMO games) but constantly moving (more like pvp in classic games).
Combat advantage damage is a big deal, as it is granted from position (behind the enemy), from "controlling" the enemy (knocked down or stunned or otherwise incapacitated for a moment) or from class specific feats and skills (gain CA for 3 seconds after doing X).
While the game has some degree of automatic targeting, the player has to constantly aim at their desired target to continue to hit them. This is basically tracking the enemy with the centralized mouse targeting icon. Switching targets takes a moment, hover over the new target for a short time to change else the game may continue to try to hit your old target. Holding down the control key will track your current target until released. Long ranged attacks can be troublesome as the constant motion of combat and tracking a target can briefly target a far, out of combat enemy, pulling a second group by accident.
Combat mechanics needs its own page to describe the effects of power, deflect, etc and str, dex, etc on damage and healing.