Except D&D went in the opposite direction with 4th edition and it is one of the major complaints about the system. Honestly, it is a tight rope WotC has to walk. My personal opinion is to air on more fluff rather than too little and include a paragraph in the DMG telling the DM it's ok to ignore baked-in fluff and make your own stuff, or - perhaps better yet - listen to what your players want the fluff for their character to be.
One of the biggest issues with the Paladin was that he DID have to follow the Lawful side of his alignment.
A paladin must be of lawful good alignment and loses all class abilities if she ever willingly commits an evil act.
Additionally, a paladin’s code requires that she respect legitimate authority, act with honor (not lying, not cheating, not using poison, and so forth), help those in need (provided they do not use the help for evil or chaotic ends), and punish those who harm or threaten innocents.
He could never topple a dictator because doing so would not be respecting legitimate authority, He would also be helping people for chaotic means because what inevitably happens after a revolution? Chaos. Sure the end product is both good and lawful but the ends do not justify the means in D&D.
Most DMs are not hardcore "Thou shalt not break alignment EVER" types but there will always be that fringe element that uses alignment like a cudgel. This is why I agree with you that alignment should be much more broad in scope and fairly vague.