Things You Should Know

For the most part, I try to stick to sourcebooks and all other officially published material for mechanics, and though I, of course, put my own spin on things, I try not to contradict any flavor text when possible. It is impossible to compile every single exception in one place, but here is, at least, an attempt.
Note that it's always a good idea to look at all relevant information on this wiki for your character's race, homeland, etc. Naturally, do whatever you want with your character; just be aware of the established standards that exist so you're not caught by surprise if your character strays outside them.

Flavor

The average heights of halflings and gnomes are swapped, making halflings the taller of the two.

The average lifespan of a dwarf is 400 years.

Tieflings can be any color your little heart desires.

The Undying warlock patron is distinct from the Undead, and can be any powerful immortal being focused on survival. Rarely good, but not categorically evil. Frequently neutral, in fact.

Souls cannot be destroyed in any way. When ghosts are dropped to 0hp, they reconstitute in a matter of hours or days.

Spells

Mage armor is visible.

Misty step is an actual step—you look like you take one step forward and sort of streak away in the direction you’re headed. Ergo, you’re pretty traceable.

The darts of magic missile appear very near the target, so it’s impossible to tell where they came from by looking at the darts themselves.

Rule of thumb should be never to assume that a spell is completely invisible. Some are, but always ask before hinging an entire plan on the assumption.

Monsters

All types of lycanthropes are inherently evil.

Brass dragons breathe poison instead of fire. (Brass dragonborn may breathe either fire or poison.)

Mechanical

A dragonborn's breath weapon is a bonus action.

As stated above, a brass dragonborn may have either a fire or poison breath weapon (with the latter requiring a CON save rather than DEX).

Polymorph is a 5th level spell with a range of touch.

The twilight domain's Twilight Sanctuary requires you to concentrate and forego your reaction each round it's up.

Firbolgs and goliaths have a base walking speed of 35 feet.

Different sources of temporary save bonuses do not stack. You get the highest one and that's it.

Feats

They are allowed, except Sharpshooter, Great Weapon Master, and Lucky.

I impose 1-2 nerfs on Mounted Combatant.

See me about...

improvements on the racial traits of earth and air genasi.

improvements on the Undying patron.

ranger tweaks (I use a blend between PHB and UA).

paladin nerfs (sorry, lay on hands and aura of protection are busted).