Kenogu kinda touched on the combat, but when you also factor in that many skills are situational and meant to be used in certain circumstances, that adds a lot of depth to the combat system. The build I primarily play, for example; A greatsword Guardian.
The first skill is a typical 3-hit auto attack that hits multiple enemies and applies might to me for every enemy the third strike lands on. This is a kind of cornerstone for the greatsword playstyle, as it's built around taking on multiple enemies at one time.
My second skill is a Symbol of Wrath that puts a circle on the ground at my location, simultaneously burning foes and healing myself and. This works great if enemies are clumped around me. I can also use this as a placed AoE heal to support allies near me as we take on the same enemies, or dodge back and place it down near a group of allies that are being attacked by ranged foes, though there are better utility skills for alleviating ranged damage against allies.
Whirling Wrath is also great when I'm surrounded by enemies, doing a massive amount of damage to every enemy around me and often hitting far-away enemies with projectiles. This skill alone, when activated while having a stack of 3-5 might after the third strike from skill 1, can kill several enemies in one go.
Leap of Faith works as a fantastic distance-closer that, on hit, blinds every enemy around me. I'll often use that move to jump into groups of mobs and start fighting them, jump back at a ranged mob after I shake off his snare to get back in his face, or get back into melee range of an enemy after dodging his power attack.
My Binding Blade is a fantastic skill that I tend to use in conjunction with Symbol of Wrath and whirling blade. It tosses blades at up to five nearby enemies with some fairly decent range, and with another press of 5, I pull all of them into melee range around me. Bust that out, slap down a Symbol of Wrath, and hit em with Whirling Wrath, and there's a good chance I have a few corpses around me when I'm done. This skill also works great when I'm snared and a ranged enemy's blasting at me, but I don't have a skill up that allows me to shake off that snare.
And it's basically like this for every class in different ways, and it becomes more and more apparent the further along you go. And All I did was explain my weapon skills; utility skills and traits open up many more venues for the combat system that you really can't grasp to its full potential until level level 20, and the elite skill is a great tide-turner, a sort of Smash orb ability that can help you snatch victory from the jaws of defeat. Like Ken said, the enemies, their tactics, and combat as a whole become increasingly more difficult and complex, and situational use of skills becomes more and more necessary.
I've never touched Tera (I'll never play a subscription-based MMO again, solely on principle), but from what I've heard, it still has mobs that go "These other two guys are really hurting me, but this guy in plate armor keeps making fun of me, so I'm going to keep attacking him instead while that guy in the robe fixes all the damage I do to him."
A break from the questclamation marks and the tired old tank/dps/heal system is exactly what I was looking for, and GW2 has delivered in spades.