It would be wrong to say that Guild Wars 2 doesn't have a resource mechanic. Tempting, but wrong. Sure, there's no real "cost" for using a skill (with a few class-specific exceptions like the Warrior's "Burst" skills), but there's a lot going on behind the scenes that you can easily miss. I know this because Arenanet's official forums are full of people missing the point. So get yourself educated as we take a look at how the seemingly simple world of Guild Wars 2 combat is actually pretty nifty.
Ok, so there's no big blue orb of resources to spend on skills, so what exactly am I on about? As I see it, you have four resources: Health, Endurance, Position, and Time. Managing these resources carefully might not be important in a pushover, but it separates the sheep from the goats when things get a bit trickier. Let's take a quick look at what I mean by...
Health: Your most important resource, since it keeps you alive.
Endurance: Used for performing an evasive roll. It regenerates slowly in combat and a full supply is enough for a measly two rolls.
Position: Where are you relative to the things that want to kill you?
Time: Your enemies are wearing you down as much as you're wearing them down. It's just a race to see who gets there first.
So to see how all of these pull together, let's take a look at one encounter, a solo battle against a Veteran Ogre Champion (we'll call him "Fluffy" for short) through the eyes of an Engineer (based on my personal experience in this exact situation). I will put resources in bold and Skill Names in italics.
My Engineer is a dual-pistoling, condition inflicting, boon-elixir-potion-chugging fiend. So when we enter the battle, I have good position and want to keep it. I set up my Rifle Turret and in order to maximize my damage before Fluffly can destroy it, I use Glue Shot to set up a puddle of glue, slowing Fluffy's advance. My turret gets a few shots off before Fluffy wails on it and I blow it up in his face to squeeze in some extra damage. However, Fluffy is now looking to get me, so I trade some endurance for some position by rolling backwards. I immediately realize that I should have instead stood my ground, blinding Fluffy with Static Shot and getting in a few shots before rolling away. Fluffy nails me with a thrown boulder, knocking me down. I've lost a lot of health and position, as by the time I'm back on my feet, Fluffy has closed to melee range. I blind him, but some bad timing means it doesn't do me a lot of good, so I have to spend all of my endurance to roll out the other way. At this point I'm stalling for time because my stun and my slow are on cooldown, so I have no way of controlling Fluffy. I use my healing skill, but it's clear that I can't outheal Fluffy's damage - I need to maximize my damage or I will run out of time.
So to help me out, I down an Elixir B for a damage boost and then get clever: I use my 9th skill to swap to a grenade kit, which replaces my weapon skills with grenade skills (while my weapon skills still recharge their cooldowns). That's how I make the most of my time! I unleash my grenade skills one after the other, then swap back to my normal weapon skills. They're ready to go, and I use my slow and blind to buy me more time to stack more conditions on fluffy and set up a new turret. However, a few seconds later I find Fluffy right in front of me about to hit me with a big swing. I'm backed into a corner, so I have to roll past him, meaning I'll hold on to my health, which is about half-full, but I don't really gain in position, as I'll still be within melee range.
I find myself running short of endurance throughout the battle, and since rolling backwards is the easiest way to gain position, it mean's I'm short on that, too. My Elixir B skill gives me a damage boost, but also a speed boost, so I have to choose between downing it and opening up on fluffy for max damage, or slamming it down and then turning to make a full retreat using the speed boost. In a sense, both are keeping me alive, as the shortage of endurance means that the occasional boulder or massive club will connect with my head, so killing Fluffy quickly is just as important as staying out of harm's way.
I opt to put some distance between us, so I have a few seconds to unload before Fluffy closes in again. However, he's still well out of melee range when I make a terrible mistake: I spam a skill. I use Static Shot just because I can, but the blind goes to waste because Fluffy is still far away anyways. He closes in, and I'm out of endurance, so I can't dodge out. I pay for my mistake with almost all of my health. A minute later I make another mistake: I blind Fluffy and then spend some endurance to dodge out of the way of his attack, even though the poor blind ogre is guaranteed to miss. That one cost me, too, but not fatally.
And after some close calls, I slay the mighty Fluffy (those two other players who came in at the end were lovely, but unnecessary, I assure you). Sure, I wasn't juggling mana costs or anything like that, but I was making cost-related choices nonetheless. I had to make the most of my skills and use them at the right time, not just whenever they were available. And rest assured, this is not unique to the Engineer. Melee classes tend to place a lower premium on position, so they can use their endurance to simply evade attacks and preserve health. Or maybe they need to pay a little health up front to wade into a big group of enemies and to make the most of a devastating area attack. The point is, Guild Wars 2 has a lot going on, but most of it is going on in the actual field of combat, not some mana pool off to the side, and I think it works.