The Miller isn't a Druid, even if he follows the Old Faith. He's a miller. Nobody goes to him when the wolves are getting too numerous, the crops need a storm to grow, or orcs are despoiling the forest. They go to a Druid. The Druid is trained in the ancient arts of the Faith. He has magic, he can shapechange. He was trained by another druid. He belongs to an ancient order of people who was trained in the same way he was. They share a common language, a common set of skills, and a belief. The miller or the knight is just a miller or knight that follows a particular faith, they aren't DRUIDS in terms of the world or game any more than I can become a minister and claim I'm the Pope.
And yet in the real world a number of Borgias were Popes. They were about as priestly as you or I, probably a lot less! In fact in a realistic campaign world the 'Great Druid' is probably a lot LESS likely to be a druid class character and a lot more likely to be some political hack. And maybe you do go to the miller, because maybe he's the guy that takes care of that stuff, and if he needs to talk to the Old Man in the Forest, then he does.
Yeah, not a selling point. I remember in the very first 4e game I ran (Into the Shadowhaunt), there is a wizard at the end with some cool spells (one I remember was called Bone-Wearying Gaze, which weakened you). Now, I had a PC wizard at the time who thought it was a cool spell and, like wizards of yore, wanted to steal and learn that spell. I had to tell him that spell was "NPC only" and he couldn't learn it. It was the first time I ever told a PC that NPCs and PCs played by different rules IN WORLD as well as IN GAME.
This old saw? 90% of the stuff that is attached to stat blocks in every edition is not reproducible by a PC, unless the DM wants to add a power/spell/whatever to the game to make it happen. So calling this a 4e problem is silly. Beyond that how hard is it to add a power to a class in 4e? Since EVERY CLASS has the same power structure its actually vastly easier than working out something similar in other editions of the game (though truthfully I don't think that's necessarily a problem, but 4e does allow for some interesting stuff if you want to go there).
Oh, a Holy Order might contain Fighters, Paladins, and Clerics, but they all don't call themselves PALADINS. That is like every person in a hospital calling themselves a doctor. A doctor has a certain set of skills and responsibilities, you can't have nurses, techs and aides doing a doctor's job. A nurse doesn't get to call himself a doctor, he doesn't have the training. I doesn't matter that they're all there to heal the sick and injured, they aren't all doctors and they don't get that title.
I think a lot of them would call themselves paladins. And maybe more to the point, some paladins might not be mainly involved in fighting, they might do other stuff. The hospital administration probably is mostly Drs. They're just not the ones practicing medicine every day, they're older, or just got into that side of things.
The warlock joins a wizard guild and realizes he has nothing to add or need when they all discuss swapping spell research and copying spells in their spellbooks and all he's there doing is drinking their mead. Fighter's rangers, and rogues all join a Holy Order and then are kicked out because they aren't adhering to the Code of Conduct a Paladin must. A cleric of the god of theft might be closely aligned with a thieves guild, but he's not sent out to do pick pockets or burgle houses. An avenger might have been a noble in his background, but he had to swear an oath of vengeance and train as a paladin.
You can have flexibility and still have the classes mean something.
I don't see any reason why the 'wizards guild' couldn't be interested in lore that pertains to things known by a warlock or vice versa. It makes perfectly good sense to me. Why can't a rogue adhere to a code of conduct? Why wouldn't a cleric belong to the thieves guild? He goes along, casts bless, detect magic, cure disease/poison, etc and shares in the loot, sounds perfectly fine to me.
Frankly I'll go further, and reiterate, I don't think NPCs largely are ANY specific class. They have abilities and whatnot that fall into categories and classes emulate those categories so that PCs can fall into them as well. So IMHO the wizard's guild isn't filled with wizard class characters, its filled with experts in magical lore of various sorts. The ways that they cast spells and do whatever they do parallel and are simulated by wizard class PCs, but the 'real' world is much messier and less clear-cut than the classes, which exist strictly to regulate PC access to different resources. Truthfully most NPCs are bit players in the game, and IMHO the older rules simply took the easy path and said "use class X for that" but if you were to detail every NPC in such a world in exact detail they'd have a wide variation and most of them wouldn't fit well into a class. Now, maybe 3.x can pretty much handle that. Note though that even 3.x in its generic rules systems generally doesn't expect the DM to go to the detail of MCing and PRCing every little NPC to make them all unique. Most of them die long before it matters. 4e is the same, you use a stat block, that doesn't imply that every NPC is that simple. Just that you don't need more detail.