So, since you didn't enjoy WoW, I have to again ask: What would you propose instead of skill cooldowns? =\
Something that would prove equally effective at limiting your usage of skills so that you have to properly time when to use it, and that using it poorly means you're unable to use the skill for about the same amount of time, to the point that you may as well have a cooldown anyways?
Mana wouldn't really fit well in GW2. Even in WoW, mana is specifically set up so that you can never run out, simply by following a rotation of skills.
In GW1, the same thing happened, except instead of rotating your skills, you'd have specific abilities designed for Energy gains that you had to periodically use. Elementalists, for example, had two spells that would cause them to gain Energy back based on the cost of the spell they just used, as well as Energy recovery built-in to their armor, as well as a glyph that reduces the cost of their next two spells.
I didn't see proper use of such spells as "skillful" considering everybody did it, and nobody failed at it. It wasn't hard to do.
Skill cooldowns is highly successful in games like LoL. LoL also has resource management in the form of either Mana, Rage, or Energy (though the latter two are rare), but all that does is force players to grab resource management items, or else frequently head back to base to regen and shop at the same time.
In the form of Rage and Energy, they don't really change up the gameplay at all (except in the case of Rage, in the case of Renekton anyways, it just serves to make his next skill extra powerful).
I have never felt that resources added a meaningful level of complexity to games.