Disclaimer: I'm not a ranked eSport arena master. And I've never made a dime beating people at a video games. But there are a few things I have learned both from experience and from talking to world class competitive PvP players. If you are willing to admit that someone knows more than you, and you are willing to listen, you can learn a great deal and improve your own game.
When making a build for sPvP, keep in mind which role you are fulfilling. On a team you need predefined roles to be effective. You also need to communicate with your team, perform your role, and trust your teammates to perform theirs.
Guild Wars 2 will release in just about 2 weeks. The first three months (or more) in sPvP will be about exploration of tactics, team makeup and builds. But during the Beta events there have been a few common team makeups and roles.
Duelist A player that excels at 1v1 combat. This role it used to solo take an objective. Usually the closest or farthest objective depending on your tactics.
Roamer This player is used to support the other roles. Quick movement is a must as this role must listen to teammates to support them quickly and turn the tide of a fight.
Point holder Traditionally this role was a player that could take a beating without dying. The goal was to stay alive solo at an objective until support could arrive, like the roamer or the team. Guild Wars 2 makes this role difficult as surviving a 2v1 is more difficult than in other games.
Team Fight This role is designed to work in synergy with 1, 2, or even 3 other players to form a team (within your team). This team is usually used to capture and hold the most contested objective such as the middle objective on most maps.
All these roles can combine in many ways for different tactics and maps. For example:
A team with a Duelist, 2 Roamers, and a 2 man team may start a match by sending the Duelist to the closest point to take and hold it while the other 4 head to the center to capture. When the battle in the center looks one, the two roamers either peel off to help the duelist, or if clear, they move to the far objective to either take it or apply pressure close to the other team's respawn.
The rest of the match for the roamers is determined by the enemy's movement and success.
Engineer sPvP builds
When you start a build, keep your role in mind. Having your skills and traits limited means you are not going to be able to fulfill them all.
To start, let's think about a Roaming build for an Engineer. Primarily you need swiftness, condition removal, control. After you fulfill those, then you can think about damage.
Well, you need swiftness to respond to team calls quickly or you can't perform your role. Condition removal and control are needed to support the person or group you are reinforcing. Chances are a solo duelist or point holder will need conditions removed. They may be close to death and need enemies peeled off with your control. When you are running to support your duelist, the duelist is likely going to still be the main source of damage, you are there to make sure he still can.
As for damage, right now the Engineer is stronger with conditions than direct damage. Engineers can apply conditions very swiftly and have a lot of access to bleeds, burning and confusion.
A bomb build
We could use a Bomb build for this. There is plenty of conditions and control in a bomb build.
So I will choose a Pistol for Confusion, Poison and Bleed.
Slick Shoes for for Super Speed.
Bomb Kit of course, it's a bomb build.
Elixir R to support teammates with revive and condition removal.
Let's choose the Med Kit for even more swiftness. (Practice using that one!)
For traits we want to go 30 into Explosives to support our bombs. Grab Forceful Explosives, Incendiary Powder, and Short Fuse.
You also want to go deep into Tools, like 20 or so so that you can get those Tool belt recharge rates down. You will be using them often.
The remaining points can go into Firearms or Alchemy depending on if you want damage or survivability. I would opt for survivability.
So there's some thoughts behind a roaming build. When filling a role, think about what you need and build around that. For a team build, you need to know how to complement your teammates with combos and filling their gaps. As a duelist you absolutely need to win your 1v1s. As an Engineer you need damage, additional survivability and some tricks. If only we could drop a Supply Crate every 10 seconds. :)