Naruto Forum

That's easy. A person with a tailed beast. They have more chakra, endurance, and unique powers. Sasuke had one of the best curse-seals and he almost lost to Naruto who was only in his first tail form.

Tailed Beast. Sasuke's curse seal only bumped him up to a level where he could match One tailed Naruto for example.  
If Naruto went two or three he would have beaten curse sealed Sasuke senseless, hell If Naruto was trying to kill him from the start it wouldn't have been much of a fight. 

Omega brought up a lot of good points through, simply having it has passive side effects that make the user more powerful without having to directly tapping into the tailed beasts abilities.