Welcome to my first post about GW2. I would like to start with something not many players really pay attention to, but which has huge impact on how a game feels, plays and interacts with player. ENGINE.
If we compare game engine with engine in a car, we can see a lot of similarities. Imagine two cars, on first sight same, but with different engines, one has a weak Smart engine and other one is like rocket. You may be able to do same things with both cars, and in most mundane tasks both cars may perform similarly, but when you want something more, the better engine kicks in. You will have no problem climbing that hill with second car, but first car (the weak one) will take ages to get up. The weak car is one slowing everyone behind you on red light, because you accelerate like a dead badger. With the rocket car you simply shoot for the stars.
Enough with car analogy (mostly because I don't know a jack about cars so I may be embarrassing myself already), lets compare engines of WOW and GW2. But before we start the comparison, please realize that WoW's engine is about 10 years old, while GW2 engine is max 5 years old. I am not bashing WoW's engine because it is bad (it is not, it's just old... Your grandpa is not bad person, just old person). And we should pay attention to what Blizzard tries to do with this bearded gray engine, because that's impressive. Thats enough for disclaimer, lets jump to comparison:
World of Warcraft:
While old, WoW's engine is well optimized, it's developers have a long experience with it, and know what they can and can not do. The problem is, as games evolve, there is more and more things WoW's engine can NOT do, however hard they try. So there can ever be only THREE kinds of spells and abilities: first kind which applies some effect on you, your friends or both you and your friends, second type, which does the same as first one to your enemy, and third group are ground targeted abilities. To clarify it a bit, you can either heal and buff yourself and your allies or use some utility, like teleport, harm and debuff enemies, or put something on the ground which either harms/debuffs or heals/buffs. You will find some abilities and spells usable only under certain circumstances, most need some kind of target to use.
This may seems to limiting combat at first glance, but thanks to well optimized engine, the combat feels slick and fluid, skills and abilities go off instantly as you press a button, your character always does what you want him to do (except when under effects like fear and stun). WoW's engine really shines when it comes down to control of your character and it's skills. Each and every ability has an animation and a cooldown. Cooldown timer limits how often you can use an ability, while animation just shows the execution of ability and is instantly overwritten with new animation of a skill used later. So animations do not affect your gameplay other than visually, cooldowns determine when you use which skill, spell and/or ability. Everything is easy to use, you click on red enemy, which makes you target him. Most spells you have can NOT be used when you are too far away from your enemy to hit him, either melee or ranged. When you are in range, all spells and attacks are guided, so no matter where enemy moves, your fireball hits him. (on miss chance later). Other abilities you have are ground targeted which are also easy to use, and some self applied effects like buffs or teleport.
As you cant run out of the way of the fireball, your chance to miss is determined by your stats (points which give % chance to miss, hit, crit, dodge, parry, block...)
Combination of guided attacks and stat based hit/miss makes for a traditional RPG setup. The engine of WoW is understandably built around this system, and it is only this system where it shines. When you start adding new game mechanics which are popping up all round in new MMOs to old engine like WoW has, it crumbles. For example vehicles in WoW; the engine is not made to handle something which takes time to turn, so vehicle combat is jerky. Flying and/or mounted combat is so jerky it forced blizzard to give additional rewards for completing Oculus, flying mounted combat heroic 5 man instance (drakes ho!). You cant really change engine too much, it's so god damn difficult to do so correctly (read: takes ages and loads of money). On the other hand...
Guild Wars 2:
Despite not playing the game, engine is nothing magical and we can easily understand how it works without actually trying it out, just how you can understand how WoW's engine works without playing with it. (Btw, you can try WoW's engine with WoW-below-20-for-free demo, which is exactly what the demo is best for.)
What I can tell for sure from countless hours of videos and comments from people who played the game is surprisingly a lot. Combat is based on proximity semi-manual targeted spells and abilities, which are NOT guided. What this terrible sentence means is there are two (bear with me!) kinds of abilities in GW2: first one which applies some effect on you and/or your friends and second kind does damage plus in many cases applies an effect on enemy or even on yourself. "WHAT? Only two kinds? BS!" I hear you scream. But under those two kinds of abilities is much more versatile and differentiated pallet of skills than in WoW, thanks to GW2 engine. The thing is, you must more or less aim your attacks at your opponent, even when using targeted spells and abilities, not just ground targeted like in WoW. You can also attack whenever you want with almost whichever skill you want without target, the game will target what your skill hit just to help you aim a little but if you target boss and attack additional small mobs, you will have no problem dealing damage to them.
Ranged combat is still a little easier in both games (just easier in wow but easier and less rewarding in GW2, you need skill for ranged too) in my opinion, so I will start there. In GW2 your character automatically aims at his opponent when you wield ranged weapon (FPS aiming in MMOs is just TOO hardcore), but all shots and spells travel the way you shot them (makes sense) and do not follow your enemy. Maybe some skills do follow enemy but that will be very uncommon and intended.
In melee, the difference between WoW and GW2 starts to really show. You have to have your front half turned to enemy to hit him in GW2 just like in WoW, but as you can use skill whenever you want, you will often miss due to swinging your sword just bit out of range, or because you stabbed air like it deserved it just because your enemy moved. You will have to pay much more attention to your range from enemy than in wow, as you can waste your most valuable skill on pesky air.
To not stop here, skills and abilities in GW2 aren't just damage X or debuff Y, not only they mostly combine those two but they are often used in different ways than in WoW. Of course you still have many spells which are cast around you and traditional ground targeted spells, there are also more MOBA-like spells (MOBA=Multiplayer Online Battle Arena, like League of Legends or Heroes of Newerth). Walls which can't be crossed, cone attacks you aim, leaps, dodges, teleports, etc. The sheer variety of ways you can use skills and abilities are crazy. Also, if your enemy is not where you attacked because he moved or DODGED you just miss.
Dodging incoming attacks and evading by moving your character is so important part of GW2 combat. This is one of things which WoW's engine is simply not capable of doing at all. Your dodging will decide how much damage you take, not stats which give you percentage chance to dodge or evade etc. If you are good enough at moving your character out of harms way, you may even kill champion NPCs 5 levels above you.
GW2 engine also have capabilities to enable many amazing new mechanics coming to MMO genre from single player games, MOBA games, FPS games and strategy games and to run them smoothly, without a hassle, because this engine is built around this combat system and with all those mechanics in mind.
A though on the end: compare WoW and Guild Wars 2 underwater combat - new Vashjir zone in WoW which specializes on underwater combat and any underwater combat in GW2. THAT is the POWER OF GREAT ENGINE!