Kurapika uses vows and limitations. The stronger the vows and limitations are, the stronger the Nen.
a student of Nen can increase the overall power of an individual skill by stating a self-imposed restriction that forces even more conditions on it.
Kurapika is the one of the only characters shown who uses such strong vows and limitations because of its risk. If he makes one wrong move and attacks somebody who isn't a spider, he will die. For most people, this is a risk that isn't worth taking because they could easily make a mistake - death does not leave any second chanced. Kurapika took on this risk because he was desperate for a way to beat the spiders.
On top of this, only being able to use your Nen on a specific group of people, especially one that is only thirteen members, limits the use of your powers substantially. Kurapika's power was one developed exclusively for fighting the spiders but most other Nen users would want to be able to use their power more generally.
However, other characters do use weaker versions of vows and limitations. For example, Bomber must tell all his victims about his powers before using it, which means he loses the advantage of anonymity.
Combined with his specialist power of Emperor Time, which means he can use all types of Nen with 100% efficiency, Kurapika's Nen becomes very powerful. Emperor Time is very useful because one of the major weaknesses of Nen can be attributed to not being the right 'type'. Killua is bad at emitting aura (Chimera Ant arc - Chapter 237 - Page 7) and this is presumably because he is a transmuter. The same with Machi.
If the threads leave Machi's fingers, their ultimate strength drops significantly.
Enhancers have the advantage of being able to enhance their power. A transmuter only has 40% of this efficiency and so on so forth.
Forcibly learning a Nen of a type that doesn't suit you can result in one losing the ability to perform other Nen skills. Hisoka calls this running out of memory.
With a combination of these two factors, Kurapika's Nen is far more powerful than most Nen users.