I disagree that the alignment restrictions of the Barbarian and the Monk should remain; furthermore, I’d like to advocate the alternative Paladins from Unearthed Arcana as core options. First, the Barbarian:
What does the Barbarian do? He gets really, really angry and this actually results in a physiological change to his body. It isn’t just throwing a tantrum (anyone can do that, and how many times does it actually help the person?), it’s taking an emotion and focusing it into a useful purpose. Psychic Warriors using the Animal Affinity power are doing practically the same thing and neither the class nor the power has an alignment restriction. So why can’t Barbarians be lawful?
Now, the Monk:
Wizards of the Coast made Monks any lawful (or they kept them that way, I’m not familiar with the previous editions) and gave the explanation “It’s because you have to have a lawful alignment to lead the disciplined lifestyle necessary to be a Monk.”
Wizards, in Complete Scoundrel, cite Riddick from Pitch Black as an example of someone Chaotic Evil (and I would tend to agree; Riddick is a murderer and on the wrong side of the law). Well, in the movie, how many times does Riddick yell out in frustration? Lose his cool? Lose his composure? IMO, Riddick is one of the most disciplined characters in the entire movie, which means that discipline is independent of the law/chaos axis, which means that Monks being only lawful because they’re disciplined is a poor reason. Either Pathfinder needs to keep Monks lawful only (but for a different better reason), or they need to scrap that alignment restriction.
Finally, the Paladin:
LG Paladins can come into existence at level 1. Why is it that alternative alignment Paladins (Blackguards, Gray Guards, etc.) don’t come into being that soon? Does no one say, “You know how Paladins are purely devoted to LG? Well, that’s how I feel about CG/LE/etc.”? Wouldn’t Rovagug, Norgorber, and Asmodeus want to have (un)holy warriors of their churches start out as soon as Torag and Iomedae?
I’d like to see the Paladins of Freedom, Tyranny, and Slaughter be just as much core as the Paladin of Justice (a la Unearthed Arcana).
I think paladins would make a perfect prestige class. The only reason they are not is that they were a regular class in AD&D.