More to the point, Good is used because it is a shorthand for specific beliefs which are significant to alignment in D&D. I consider myself "good," but in D&D terms, would probably rate a solid CN. While I could debate for hours or days with someone about what "good" really is, it's usually pretty straightforward to discuss whether something is Good in D&D.
Playing a Good character does not, however, tell you whether you are playing a Kantian moralist (treat others as you would for everyone to treat all others at all times) or a utilitarian one (like a Vulcan, the greatest good for the greatest number). It doesn't specify a Christian medieval good or a Buddhist one. It means precisely:
Good characters and creatures protect innocent life. Evil characters and creatures debase or destroy innocent life, whether for fun or profit.
“Good” implies altruism, respect for life, and a concern for the dignity of sentient beings. Good characters make personal sacrifices to help others.
HOW you go about doing that has endless variations, and Law vs. Chaos is just the beginning (with the nested question of individual versus social codes of behavior).
Playing a Paladin is hard, simply because they cannot drift in alignment. Not only, unlike most characters, must they maintain a specific alignment, but they cannot perform any evil acts whatsoever. Whereas a LG wizard might occasionally perform an Evil act if pressed.
Now, demanding that a Paladin not perform evil acts shouldn't be any more demanding than what you would define for any other character. Killing a captured orc is not a Good act, but it's not unquestionably evil, either, if you think the orc represents a threat to others. I think where people trip up is thinking these kinds of hard moral dilemmas is a fulcrum on which a paladin's powers may shift at any moment. Not so. Hard moral dilemmas are dilemmas, and as long as the paladin attempts to make a good choice, he is not in any real moral danger.
A paladin loses his powers, not for simply lacking a good answer to all of life's evils, but for committing an act which he should not commit. A paladin could even challenge another Good character and combat and kill them, if need be; an honorable challenge is not disrespectful of life, nor is an armed opponent an "innocent." However, a paladin could not be a professional gladiator, killing slaves or financially desperate opponents, or battling for petty honor. But a paladin could sponsor a war of conquest, if he felt the greater good truly justified it.