How did you get those kinds of numbers at 2nd level? I'll admit, I don't have essentials, so, I'm still stuck with the standard Rogue, but, +5 for stat bonus, +3 for dagger, +1 for Rogue using a dagger +1 for level and +2 for CA=+12. Where's the other +5 attack bonus coming from?
And, how do you do 3d8 for backstab and not 2d8, even with the feat? Or is this an Essentials thing?
Essentials - Essentials Thieves I think are a good bit more powerful than PHB Rogues at low level. It seems to even out later. The books the GM allows are Essentials, PHB, and PHB2. Larsenio is built with Heroes of the Fallen Lands + Backstabber feat from PHB.
Tricks: Ambush Trick gives CA & thus sneak attack vs a lone enemy. Tactical Trick gives CA & sneak att vs enemy with one of my allies adjacent.
My only magic item is a level 1 magic dagger, which is turning into a signature item as I'm using it in preference to eg shortbow for ranged attacks - less damage, but very convenient not to have to keep swapping weapons.
Larsenio's Sneak Attack:
Attack: +5 DEX +3 Proficiency +1 magic +1 Thief Weapon Talent +1 Level +1 Light Blade Expertise +2 Combat Advantage = +14
Damage: +5 DEX +1 magic +2 weapon finesse +1 Light Blade Expertise with CA = +9, +2d8 Sneak Attack with Backstabber feat, 1d4 Dagger = 1d4+2d8+9
Total: ATT +14 dam 1d4+2d8+9
Backstab 1/encounter adds +3 to hit, +1d8 damage, for ATT +17 dam 1d4+3d8+9
If I miss I can Heroic Effort 1/encounter for +4 to hit.
My Init +10 is DEX 20 (+5), +1 level, +4 improved initiative.
Note that I put everything possible into maxing out my to-hit bonus, on the basis that a Striker absolutely has to be hitting to be effective, and there are 2 other PCs the monsters focus on - our defender Fighter and especially the minotaur melee Ranger with his AC 14 is just too tasty a target for them to pass up.

My damage is not maxed out in that a shortbow would do more damage (d8 instead of d4) but I don't have money for a magic shortbow and I think the dagger fits my character concept better anyway.
I used to have Toughness, but I swapped it out on levelling since I was never being attacked. At 1st level my sneak attack was ATT +11 dam 1d4+2D6+8. On reaching 2nd I swapped out Toughness for Backstabber and took Light Blade Expertise (ATT+1, +1 damage vs targets you have CA against). And +1 for Thief Weapon Talent kicks in at 2nd in Essentials. So levelling up increased my ATT +3 and my average damage +3 too.