Theres good ones. Awesome ones and bad ones. Most games with 100% completion only gives you a longer ending while others give you a item/weapon witch you can only get for getting EVERYTHING in the game.
Examples:
If you get all of the stars in super Mario 64 then beat ultimate bowser again, He will say saything else.
Getting 101% in donkey Kong 64 will get you a VERY BORING ending then the normal ending.
In most Legend of Zelda games, Getting 100% wont change the ending or get you a new area/item. But the hardest Zelda game to get 100% on is Wind Waker, You have to take photos of every enemy and character and you can only hold up to 3 at once, there are OVER 300 CHARACTERS in the game. Good luck with that.
Getting all 100 jiggies in Banjo-Kazooie will extend the ending, Mumbo will say some secrets in the game and talk about the next game, Banjo-Tooie.
Spyro 3 had by far the best 100% Award, If you go to the last lobby area, You will find money bags, the greedy money bear. You meet him throughout the game to lower a bridge, Rise platforums and other stuff. He will say he has a egg and is going to sell it back at his home world, He will then say you are looking at him weird. He then makes a run for it, Charge into him multiple times until you get every dam gem he took from you.
