After attaining the pleasure of pushing each class to level 15(not hard at all once you reach soul rank 7) I have come to the conclusion that thief is the most "complete" class to play as in the game. The thief is great for soloing, parties, and PvP. A thief can be designed to play any of the major roles in a party.(DPS, TANK, HEAL,SCOUT) In short, a team of 4 thieves can take down any content thus far(Dragoon Ruins) with ease. 

When looking at the thief skills, you notice that it has 3 distinct branches. Trickster, Sidewinder, and Strategist.
This branch is the "combo" type build. It allows for quick blows that are very strong. OD recovery is a must if you plan to be in the monsters face, it allows you to spam the hide attack(best utilized on large, slow enemies so you can always cast on their back). The "Master Skill" gained at lvl 30 seems very out of place, not really useful for an attack build thief. Although I wouldn't recommend this build to starters, with skills from other classes this becomes an extremely deadly build(also the leveling content is not yet available[Dragoon Ruins Cap] to reach the higher skills so you arent just spamming hide attack and waiting on warzen blow to cooldown). This build is very doable even from early on if you take advantage of the Hate Decrease passive, this will allow you to deal more damage than the rest of your team and still have exclusive access to the backs of an enemy. In short this build would be defined as the DPS type.(dipping into the strategist tree further increases this builds viability as well)
With a few points in stealth(strategist tree), this is the tree that can take down ANY enemy with ease. Save a few points to max out Cut/Bomb trap enchancements and by lvl 12 you will be one tough thief! This build takes a bit of time to get used to. You are going to have to do quite a bit of running from enemies and resetting their aggression with stealth. Also to maximize trap damage DEX/STR does help...but ultimately your weapon physical attack power is the main factor in how much damge your traps deal. Taking over attack boosts from other classes will increase trap damage. HOWEVER taking traps to another class is not advised unless you plan to grandfather them back in to your thief later on. The thief class has a passive class skill that GREATLY increases trap damage. This makes trapping only viable as a thief.(Please feel free to argue traps viability as another class in the comments section below) Now to make the most of all the enhancements to your trap be sure to circle around your trap as it is going off, to ensure the enemy chasing you gets hit by each AoE.(Bomb tends to miss fast moving enemies if this isnt done, which is why I suggest putting points into CUT before BOMB if you ever come across a point where you arent sure which one to level first)
This is the golden branch of the thief skill branches. Every single class in the game can benefit from something  in here. Stealth and Decoy are THE skills everyone can make use of. Im sure most already know, but a level 7 stealth only has 10 seconds after the skill runs out until you can recast it. Level 7 Decoy is just as good, this sucker can tank a boss for AGES. Must have skill, makes instance fights a cake walk. Steal Attack at lvl 7 is also a pretty handy skill to have endgame...I'll leave it at that. AS far as the chest opening skills go, I'm yet to play around with them too seriously but I can imagine if you are rolling a porkul with 10 in LUK just to be a DISTRACT/LOOT addition to the party they will be very handy. Make medicine is also a very handy skill, as you level it the the cooldown is reduced significantly.(I wouldnt suggest investing more than 2 points in it, personally I just put 1 in and try to use my potions sparingly) Im yet to invest any points in medicine shower or  high medicine but I do hear medicine shower is the only heal that won't add criminal points. 
Of course each branch on its own is lacking something, and you really should take from all branches as well as from other classes through the class changing process. This is one class that benefits heavily from grandfathering a skill.(I'm going to add a wiki page on grandfathering because quite a few times in game when I mention it I get a few question marks)
As I begin to play more as a thief I will describe some of the various skill builds I come across. I am providing my opinions and knowledge in hopes that others will come out and we can maximize the already powerful thief class. Please leave a comment if you appreciate this blog post and I will come up with more!

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