Basically what ACG said. You should look at it like this:
1) Burst of Thorns rolls MORE dice in proportion to its cost than most attack spells.
2) It's not an attack spell requires your opponent to counter it in a different way (if they even see it coming)
3) It has a 50% chance for a bleed, which means its damage potential is even higher, easily being at a 1:1 damage to mana ratio or higher (better than most attack spells).
4) Free piercing +1.
5) It is a RANGED attack, hence, bypasses guards, which is HUGE.
This spell is simply one of the most powerful relative to its cost.