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If any of this sounds simple, well it was designed by ArenaNet to be intuitive. However, I hope this guide helps the confused or overwhelmed newcomer. Take your time developing your build, your knowledge matters in this game.
So, you want to play profession A, and want to be the best in whatever you play. As most things are, Guild Wars 2 is not that simple. To be good, you are going to need to choose your profession, select your weapon set/s, find the right utilities, evaluate your trait options, and then finally compatible accessories, sigils, jewels, and runes. It can be all a little overwhelming, but I'm here to help you through it.
Now before you dive in, you need to evaluate what you want out of your build, and what you want to do with it. Do you want to be a glass cannon in competitive 5v5? Do you want to be play a support character to help your realm to victory in WvW? These all matter, and need to be well-thought out in advance to making your build. The order of this article is ordered purposely, going from broad to very, very specified. I followed ArenaNet's own advice, this was ordered the exact way you are introduced to these systems in the game. ArenaNet's design was intentional, if you do it this way, there will be considerably less frustration later on.There is an order to doing all this, but once you have it down, you can start making builds extremely effectively.
First focus on the weapons. They are the most vital piece to your gameplay, and how you play will change dramatically based on this first choice. Ignore underwater weapons for now, as they are significantly less important than your on-land options. You need to keep in mind how many weapon sets your profession can equip. To put it simply, as an engineer or an elementalist you only have one option, so you need to make it count. If you choose the others, you need to make sure that the sets synergize well together. If you are going for a WvW build, you probably don't want two melee weapons, unless you know what you are doing. Most players will carry a melee and ranged weapon set, as this will allow them to adapt to the most situations. After the initial months, specifically in competitive PvP, this will probably not hold true. Many people will try to fit into niche roles to complete successful and unique team builds in ranked games even if that means stacking two ranged sets, etc. Keep track of what survivability the weapon has on its own, as well as the conditions provided. For example, if there's a lot of moves that add the condition bleeding, then you may want to add traits later on that enhance that condition.
If you are following me so far, you now have your weapon or weapons. Next are the utilities. There are 3 heals to choose from to fill your #6 slot, 20 utilities to choose for your #7,8,9 slots, and a choice of 3 elites for your #10 slot. Yes, there are racial elites, but unless you are trying to roleplay, you will want to avoid them; they are designed to be worse than their class-counterparts, and are not usable in structured PvP. Please keep in mind here about the upcoming section on traits. You may want to think about getting a similar group of utilities, for example getting two of one type. Maybe two signets or two skills of one type like Glamour or Deception. You can find the skill type of each utility on the ever-useful Guild Wars 2 wiki. Many traits will provide 20% reduction in cooldowns of all signets, or longer lasting stealth moves. You need to keep this in mind. By stacking a certain type of skill, you are creating synergy in your build. The most effective builds will have the most synergy. However, don't place all your eggs in one basket. In the last beta, one-skill wonders weren't effective. Most people were relying on balanced builds, ones that had some offensive skills, defensive skills, etc. If you have two really nice offensive signets, and need a defensive ability, and the defensive signets are just not your style, then don't feel obligated to choose a 3rd signet. Go out and synergize somewhere else that may help you indirectly. If I'm a stealth-based, backstab thief, I can't just maximize damage and be effective. I'm going to have to have some immobilization or control skills, that allow myself to get behind my opponent.
Traits. Traits are the place where pro's are going to differentiate themselves from the newcomers. Every minor and major trait matters, and you need to plan ahead. You will not want to waste 10 points to get to a major trait, when the minor trait is useless. If you know what you are doing, then you definitely can, but that major trait better be critical for your build. To do this effectively, at this point, is a little bit hard. Many of the abilities say increased damage or provide some benefit that is written about obscurely, but we do not have hard numbers. Due to this simple fact, you will be unsure about choosing that several traits. I want to say here that it is okay. It's okay to not know, it's pre-release for God's sake. Take your time with this section, as it is the most complex. Keep in mind that the attributes also matter. Each tree has a maximum and minor attribute that can be utilized to maximize the efficiency of your build. Maybe you really like one of the defensive trees that provide Toughness, and you can now go back and make three offensive utilities.
You not only have to micro-balance among each these sections, but your overall build should be balanced as well. If you do traits right, your work on the battlefield will be a lot less harder. If you are sloppy here, your fights will be an uphill battle. This game not only maximizes your skill as a fighter but your skill as a tactician. Knowledge is power here, however, this is not Buildcraft. Skill and knowledge will be needed, but a great build is half the fight.
I will continue onto accessories, sigils, jewels and everything else in part two. There may be some delay in part two til after the next beta, because I have not been able to experiment with these myself. If you focus on weapons, utilities, and traits, 90% of your work is done. To get that last 10%, you are going to have to work for it. Your build should now be pretty specified, and the rest will either add to or balance your build.
If you are excited to venture forth and make your build. Use this resource, its almost completely up to date, and very easy to use: http://www.gw2tools.com/
My Guild Wars 2 Blog can be found here: Divinity's Reach