Classes and Subclasses

Mostly, the rules and descriptions of the classes and subclasses from the PHB stand. There are some exceptions, however. Some are for thematic reasons, some for gameplay reasons. If a subclass is not mentioned below, then nothing particularly unusual pertains to it in this campaign.

Barbarian

Barbarians call upon the spirits associated with The Old Way to gain any supernatural powers (more here).

Bard

Explaining the house rule on Magical Secrets: If that feature is used to take a spell that is exclusive to the Paladin or Ranger lists, then such a spell counts for all intents and purposes as a level higher than it normally is. It can only be chosen if the bard has access to that higher level of spell, it costs a higher level spell slot to cast, and it interacts with other rules as being a level higher than it would be to a paladin or ranger.
There is no strong representation for the Colleges of the bards in Chepdale, unless the players make them.

Cleric

You can get more information about the gods here. If clerics go against the ethos and alignment of the god, pantheon or way that grants their spells, they begin to suffer a removal of their powers. Initially such a penalty would be temporary, but if the cleric fails to do better, it can become permanent. Clerics are supposed to lead by example, and are held to a high standard by mortal and immortal alike.
The domains you can choose from depends on which religion you follow. Follow the link on the gods for details on which domains go with what faiths. There are temples to Tilaus and Yanna in Chepdale, and a circle of druids following the Old Way in Nan Mehtar.

Druid

Moon druids cannot wild shape to beasts higher than CR ½ until level 5. Turning into a brown bear at level 2 is badly overpowered, and therefore this nerf.
Druids follow The Old Way, gaining their power through their understanding and acceptance of the balances in the world. As a consequence, druids tend to have a neutral element in their alignment, although this is not a requirement.
There is a Circle of the Land active in Nan Mehtar, and they will no doubt be happy to cooperate with druids of any circle whose aims align with theirs.

Fighter

If you intend to become an Eldritch Knight, please be sure to include some spellcasting training in your backstory. Int-based spellcasting doesn't generally spring up unexpectedly.

Monk

Monks clash with the themes of the campaign world and do not belong in it. Hence they are not allowed.

Paladin

Like clerics, paladins gain their powers through a special relationship with one or more supernatural beings. You should choose a faith to follow from the beginning. The Oath a paladin takes at level 3 should work well with the chosen patron.

Ranger

Rangers call upon the spirits associated with The Old Way to gain any supernatural powers (more here).
The Beastmaster Ranger seems too bizarre to be allowed. The way the ranger constantly needs to command the companion makes no narrative sense, although I must admit it seems quite mechanically balanced.

Rogue

Don't steal from other PCs, and don't antagonize the 'good guys' in Nan Mehtar. That is not how we play this campaign.
If you intend to become an Arcane Trickster, please be sure to include some spellcasting training in your backstory. Int-based spellcasting doesn't generally spring up unexpectedly.

Sorcerer

Wild Magic is thematically problematic and can be quite annoying mechanically. And dragons don't interbreed with other species in this campaign, hence no dragonblood. So the two basic subclasses are out. You can instead use the High Elf Bloodline, the Favored Soul (scroll down quite a bit, or use a search tool), or Storm Sorcery from Sword Coast Adventurer's Guide (p. 137). The Favoured Soul is chosen by a specific god or pantheon; the Old Way does not have Favoured Souls.

Warlock

Only the Fey pact warlock is acceptable as a player character in this campaign. The other two kinds exist, but are villains, not heroes.

Wizard

Illusion and Necromancy wizards come with various problems and are banned from PC use. Spells from the schools are not banned, but don't expect to command undead in this campaign. Animating corpses is withholding the spirit of the deceased from its final resting place, and an act of unmitigated evil. Illusion spells that allow great player creativity also cause a lot of DM head aches and breaks in the game. They are not banned outright, but don't expect to build a character around them. Consider them on a watch list, if you like.