Luke's powerful himself. Having the last name Skywalker means a lot...Anyway, do you think Darth Vader was really going full throttle on his son? No he felt for him so much that he even saved him after Luke cut off his hand.
Also did you even read my post above. It explains Form V, which is the Form Luke uses the same as Vader...
"Form V maintains its existence through having sufficient defensive skills, as derived from Form III, but channeling defense into offense. The example is commonly used that while Form III combatants effortlessly deflect laser bolts, Form V practitioners excel at redirecting the laser bolt toward the opponent. This simultaneously defends the user and efficiently injures the opponent. As well, it employs a basic form of Form II's parries, allowing the user to parry the attempted attack, and counter it. It is different from Form III in the fact that practictioners of Form V will press the assault, using wide, sweeping blows in an attempt to overwhelm the opponent with brute strength. The aggressive philosophy of Form V is the source of many a Jedi's disapproval.
Vader also created his own variant of Form V, where he would use only one hand to strike and defend, and have the other held casually by his side. This can be seen at the start of the duel in The Empire Strikes Back.
With the aggressive but yet refined movements of Form II and the highly defensive postures of Form III, Shien / Djem So has been proven to be an effective style."
The Form is used to overwhelm the opponent with brute force and wide swings. That explains Luke's nature with a lightsaber.