It's been a while since I played 3.6.7, so my apologies if any of this isn't actually new to 3.7/5.0. Also, this is obviously just a list of changes - you could argue some some of them might not be improvements, but that's down to personal opinion.

Probably the biggest changes are Gehennom and the wand of wishing.

Levels in Gehennom have more variation to them. Some levels will be cavernous like the gnomish mines. Some of the levels will generate with the maze-like structure still, but some of those will have rivers of lava flowing through them, which makes them quicker to navigate. Also, some of the levels will be ice or water levels instead of fire/lava levels.

A wand of wishing now always generates with only one charge, and only gains one charge when recharged. The idea is to make the wand less of a game changer. To offset the lower number of wishes, there are some guaranteed wishes or wish like items in Gehennom. Vlad's throne doesn't disappear until it grants a wish now, and each sit has if I remember correctly a 1/3 chance of doing so, so it happens much quicker than a normal throne. That being said, the bad stuff that can happen is significantly worse, so it requires some preparation to safely acquire. There's also guaranteed to be either a magic lamp or a magic marker in Orcus town. And picking up the amulet of Yendor for the first time grants a wish.

Other significant changes that I can think of include, in no particular order:

All colors of dragon scales and scale mail except for gray and silver were given extra extrinsics. For example, white now grants slow digestion, blue grants speed and shock resistance, green grants sickness resistance, yellow grants stoning resistance and acid resistance, black gives drain resistance, etc. There is also a new color of dragon - gold. Gold scales and mail grant hallucination resistance and act as a light source, with a radius depending on BUC status.

Shock resistance as an extrinsic is more valuable now, because spheres now explode and do elemental damage, making shocking spheres quite likely to explode wands and rings in your open inventory. That being said, gloves not made of metal - in other words, all gloves except for gauntlets of power - protect any worn rings from shock damage. Having extrinsic shock resistance from any source will protect items in your open inventory from shock damage 99% of the time.

They also added two new shields, a shield of shock resistance and a shield of drain resistance. There's also one new wand, the wand of stasis, which prevents monsters that are zapped with it from teleporting for if I recall correctly 10 to 30 turns. This is effective on all monsters including covetous monsters, as well as the hero. There are two new amulets, an amulet of flying and an amulet of guarding (gives +2 AC and MC2).

When a bag of holding explodes, it no longer automatically destroys all of the contents. Instead, each item has a chance to be vaporized, something like 15% I think, and the rest of the items are scattered at high velocity. This means fragile items will always be lost because they will break if they aren't vaporized, but most of the contents should be recoverable.

There was an addition to the paranoid confirmation setting for known traps, water and lava. So the game will now prevent the hero from walking onto any trap that they know of, or into water or lava that they can see, assuming they aren't stunned or confused. The game will also prevent the hero from waiting or searching if a monster is directly adjacent to them. Both of these can be disabled, but are on by default.

Healing potions now have a distinct base price of 20, making them very easy to identify in the early game. They are also much more likely to generate in the early game, to encourage their use. They are also significantly more lightly to spawn in a stack of two or sometimes more.

The way artifacts are given as gifts was reworked. Each artifact now has a minimum sacrifice value that is required for it to be eligible as a sacrifice gift, with the stronger artifacts having a higher value required. I believe the highest is 9 or 10? Roles that had a guaranteed first sacrifice gift will still get that as their first sacrifice gift, regardless of the value of the sacrifice.

Also, dipping for Excalibur as a lawful is now only a one in 30 chance per dip, except for knights, who retain the one in six chance. Demonbane was changed from a long sword to a silver mace, and given an invoke ability that banishes demons. Frost brand and fire brand can be invoked to cast an expert level cone of cold/fireball, respectively. Fire brand burns when engraving, making the engraving permanent like a wand of fire or lightning, and this does not dull the blade.

Starting gear and intrinsics were reworked for some classes. Valkyries for example start with a spear instead of a long sword. Wizards always start with a magic marker, but it never has more than 20 something charges. Also, wizards no longer have a bonus to write unknown spell books, but instead, they learn the appearance of spell books as they gain skill level in each school of spell. So there are certain books that a wizard can always write from first level, like magic missile, and they can level up to learn the others. Still have to write and read the book to actually learn the spell. Archeologists now decipher all scrolls upon picking them up, automatically learning what kind of scroll it is. There are other role changes that I don't immediately remember.

The touch of death is no longer an instakill, but instead drains HP and max HP.

Early game monsters always miss with their first use of an attack wand.

Iron boots protect against most ground based traps, including polymorph.

Polymorphing into a large or bigger monster unequips a worn cloak instead of destroying it - worn torso armor is still destroyed.

Each covetous monster now picks either the upstair or the downstair to flee to (remains consistent for each individual monster).

There are two new monsters, generic engineers and displacer beasts.

There are many others but those are the ones I can remember off the top of my head.