One of the most common misconceptions about alignment, in my view, is that Lawful Good means "accepts the law of the land," whatever that law may be. I've even seen it commonly suggested that a Lawful Good character faced with an unjust government would be conflicted between standing up for what's right and preserving the existing order. I don't think that describes Lawful Good at all. (But even the current iteration of the playtest rules to seems to reinforce it with "as expected by society.") Conversely, Chaotic Good doesn't mean simply "rebels against the existing establishment." I find it odd that Robin Hood is always used as a Chaotic Good example. The most common version of Robin Hood is rebelling against a usurper in an effort to restore the true king! That's Lawful Good.
So here are what I think are accurate and succinct descriptions of the alignments. Framed right, I think alignments really can provide a valuable roleplaying guide. I'd like to see them fleshed out more in the rules, and playing a larger role in character customization.
Lawful Good: Interested not only in morality, but in justice and order. Order has a value, and the best way to accomplish good is through a just society and laws. They strive to bring about a lasting good society or community, not just to uphold the current order.
Neutral Good: Primarily concerned with morality. Sees the value of order, but will always place meeting others' needs above concerns like just punishment for wrongdoing.
Chaotic Good: Actually feels chafed by rigid rules and societal expectations. Seeks to help others mainly by freeing them to make their own decisions and determine their own values.
Lawful Neutral: Justice and order are the primary concern. No particular compulsion to sacrifice of themselves for common welfare; would have compassion mainly for people close to them.
Neutral: Most people. Stays more or less within the confines of society's expectations, though not based on principle. Looks out for their own, but not really interested in helping others or seeking moral betterment.
Chaotic Neutral: Again, chafes under authority, rules and restrictions. Primarily concerned with their own freedom to do as they please, not with anyone else's.
Lawful Evil: Interested in seeking their own ends and increasing their own power, and most comfortable doing so in an orderly system. Desire to bring others under their control and to their way of thinking, and will sacrifice others' welfare toward the purpose.
Neutral Evil: Totally out for themselves, and will twist laws, government structures, or lack thereof to their best advantage. The most unscrupulous kind of person.
Chaotic Evil: A bully. Concerned with their own gratification - usually their immediate whims. Don't like people telling them what to do, and will enjoy causing harm to anyone who angers them or gets in their way.