Well, if Naruto had a Green Lantern ring, he would be extremely powerful. His already formidable willpower from his ninja training would be amplified by the ring. He could create constructs of his shadow clones, or use the ring's energy to enhance his Rasengan. It would be a whole new level of combat for him, combining the powers of the ninja world with the abilities of the Green Lantern Corps.
His mobility would be enhanced. He could fly around like Green Lanterns do, which is a huge change from his normal ninja running and jumping. He could also use the ring to create shields, protecting himself from attacks that his ninja gear might not be able to handle.
It depends on your taste. Some people love the imaginative universe and complex plots of Green Lantern comics. However, if you're not into superhero or sci-fi themes, you might not find them as good.
Definitely not. John Diggle and Green Lantern are distinct entities in the comics. Green Lantern has specific powers, origins, and adventures that are separate from those of John Diggle.