- 2nd Edition Player's Handbook
Neutral Good characters, I think, should be idealists (more idealist than the straight-as-an-arrow Lawful Good types) - people who have Dreams, who want to achieve Peace on Earth and Goodwill To All Mankind, and who will stop at nothing in the pursuit of What's Best. In a modern context they would be animal rights campaigners, peace activists, international volunteers and freedom fighters. In the medieval high fantasy settings typical of AD&D, they might be Knights Errant, undertaking quests for the betterment of mankind, peasant heroes standing up for the underclass, or wandering healers. The do-gooders and starry-eyed of whatever world in which the game is taking place.
Unlike True Neutral types, Neutral Good characters make natural adventurers, especially if reasons are contrived: Bob the Knight Errant goes on adventures to raise money which he can distrubute to the poor; Gwenda the Healer enters the dungeon to try to find artifacts to make her work more effective; Job the Cleric goes to the goblin lair to end their privations on the poor people in the valley.
Once again, I find myself wondering whether 'Neutral' is the best choice of term for the diametric system. The alignments make more sense when they are thought of as Lawful Good, Lawful and Lawful Evil; Good, Neutral and Evil; Chaotic Good, Chaotic and Chaotic Evil. Thus the idea about Balance can be left to the philosophers and the DM where it belongs, and characters can just have personal motivations, as we would expect them to. But such are the rules that were given.