Theyre 2 ways to completing game: Finishing the story mode/ main quest, and then theres finding,completing, and unlocking everythign 100%. That is a completionist. How many games have you completed 100%? I try to complete every single game i own 100% which is why im still playing all 8 from the holidays. Sometimes its good to play a game 100% cause then when your done you dont have to wait as long for the next game you want to come out. Here are some of the things i have to do

Find all item boxes in sonic unleashed ( wii version)
Complete the arena mode with all helpers ( Kirby superstar ultra)
Find all rings, chao eggs, and quests ( Sonic chronicles the dark brother hood)
Complete all missions and find all treasure ( wario land shake it)

Also for any xbox game, the term completionist can be considered " achievement whore" as i have been called that for actually getting 1000 points in some of the games.