Charlie had become so annoying for his insecure, envious, basically loser behavior. Trying to save Aaron by scaring Claire to death and assaulting Sun just to help him get back at Locke. Not to mention just acting sullen and sulky, one of my favorite characters from the beginning had become an eyesore. Well, there's nothing like sacrificial death to help get you on the audience's good side again, and the episode Greatest Hits a is a moving prologue to what will be Charlie's greatest moment in the season finale, willingly giving his life for the rescue of Claire and the other survivors.
This episode moved me primarily because it's powerful to see someone quietly accepting their fate, their death. In fact, the hero in Charlie finally comes out as we see that dying for the ones he loves is something profound for him -- he has finally found his role in life.
It's powerful to see someone who had once been a good person destined for greatness, who then became something of a fool, find themselves again.