Sword/Dagger is something different, it's a different way to play a Thief in GW2. It's a mix between D/D and S/P, but nothig like any of the two. You will not deal huge burst damage like in D/D (at the cost of being extremely quick and precise with your fingers!) and you will not have a nice constant damage output with a little control on your target like with S/P (at the cost of having to stand in place while swinging your sword for a couple of seconds!).
S/D is about mobility. Stealth may be useful, but not in my build. I focus on Initiative regen and Dodging.
This is my 0/30/0/25/15 (S/D with P/P as ranged) . Let's see what is in there and why.
Deadly Arts (0)
Would use it for a D/D, or maybe for a nice opening with S/P. Mug gives Steal a solid reason to be used. But in my build Steal is not needed for the opening, but during the fight if Initiative is getting low. So, no points in here.
Critical Strikes (30)
I guess this is a must for every Thief. Precision is extremely important for us. I use Side Strike for a higher crit chance when hitting from the side or behind, Practiced Tolerance for a nice health bonus (good in PvP), Executioner is a must unless you use D/D (in this case use Hidden Killer). There is not much to say, since you play Thief, you should already know everything about these traits.
Shadow Arts (0)
This is the Stealth line. As I said, many S/D build would use these traits, but I like to play in a different way, so no Stealth trait for me.
First of all, you should have at least 25 points in either Shadow Arts for Toughness or in Acrobatics for Vitality. Some theorycrafters say that Toughness works better for Thieves, but since I need the traits in this line, I decided to go 25 points here. Power of Inertia and Quick Recovery are my choices. You will evade (V) a lot if you play with this build, and evading will give you some good bonuses. So why not to use them! If you dodge you will get: a 10% damage bonus till your endurance bar is not full; one stack of might for 15 seconds; 2 seconds of swiftness. Also, dodging will use less endurance (as part of its cost it will be returned as you dodge), so you will be able to dodge more.
This is probably the unusual part, since most player would rather use these 15 points in Shadow Arts. But as I said, I focus on Initiative and dodging, not on stealth. And here I can get 3 more Initiative points (13 instead of the standard 10), stealing that will return me 3 Initiative if needed, and last Thrill of the Crime that will give Fury and Might for 10 seconds. Fury gives a straight 20% bonus to critical chance (20%!!!).
I use Hide in Shdows as healing skill. I like the higer heal from it and the short regen afterwards. I tested also Signet of Malice, which is not bad, but I'd rather have a strong emergency button.
Signet of Shadows and Signet of Agility for the bonus they give, and Blinding Powder as escape button (but also Roll for Initiative is very good). Elite skill is Thives Guild.