I feel the paladin's code is fairly well defined:
• Do not willingly do evil.
• Respect legitimate authority.
• Act honorably.
• Help the needy.
• Punish the wicked (those who threaten or harm others).
This is "well defined"???
1) The first condition uses the term "evil", which philosophers, both religious and otherwise, have been debating the meaning of for thousands of years and still haven't come up with a firm conclusion
2) The second condition uses the term "legitimate authority". Apart from "legitimate" being dependent upon what you regard an acceptable set of laws to be, this is still contested in several regions of the real world today. Just as a (fairly non-contentious) example, in medieval times who was the "legitimate" king might be determined by who was born when with the old king as their father. If opposed by a democratically elected "peasant council", such a king would be seen as unequivocally "legitimate" whereas the council was not. In the modern world, or based on modern sensibilities, I imagine this might not be quite so clear cut...
3) "Honour" has been a cornerstone of warrior culture all across the world through the ages. But what it meant to a samurai, what it meant to a knight and what it meant to a noble Parthian were very different things...
4) "The needy" - hmmm. Would those be the ones demonised as "slackers and welfare cheats" in the gutter press, or the ones who would go hungry without support - oh, wait; those could be the same people! Or not! Who gets to decide what "needy" means, again?
5) "Those who threaten or harm others" - does that mean you have to punish them without hurting them? Or do you just have to punish yourself
after you punish them - hey, clever get-out!
Overall, these don't seem at all well defined or clear, to me. They seem exceedingly subjective and fuzzy.
And there's some more rules about palling around with evil folks, but that's not a huge deal.
Right - the bit about "never associating with evil characters". And suppose they lie to you? Oh, maybe that's what the "Detect Evil" power was for. And subjecting everyone you meet to what amounts to an insult and a gross invasion of privacy is "lawful" and "good", we assume?
I don't get the problems people have with the paladin because it's very clear-cut. The only time that you're going to run into a problem with this is douchebag players and douchebag DMs, and they're going to be douchey without the paladin's code of conduct.
In the sense of "douchbag" that translates as "those who have a different view of what a range of ambiguous and contentious terms mean than I do"? Because all I see as required for problems, here, is a simple and legitimate difference of opinions.
As to "what is different with the 4e approach", it's simple. In 4e, exactly what you will be "rewarded" for and what you will be "punished" for is perfectly clear up-front (assuming you actually read the rules). In earlier editions, what you will be punished for (there being no specified reward) is entirely dependent on the world-view of one individual who is not you. This makes the "right answer" that will lead to you
not being punished essentially a mystery that you get to take a guess at. Being rewarded for guessing right is superficially fun, for a while, but being punished for guessing wrong gets old
really fast.