Returning player. Anyone care to give a quick rundown of all that's changed since Morrowind?

Freeflyer212

So, I started the game back in 2016 and by Feb 2017 I completed roughly 85%. I took a break and well after seeing a deal this past new years, I went ahead and purchased the newest expansion. So , any players willing to share with me a quick run down of all that's changed since Morrowind?

VaranisArano

This won't be exhaustive.

Morrowind: introduced the Warden class and PVP Battlegrounds, a 4v4v4 arena with a variety of gamemodes.
ZOS made major changes to CP trees and sustain. Light attacks are primarily for dealing damage. Heavy attacking restores resources based on the type of weapon.

Summerset: introduced Jewelry Crafting and the Psijic Order skill line.
Light attack damage now scales with your max stamina or max magicka depending on your weapon type, so light attack weaving becomes an even more important component of DPS.

Elsweyr: introduced the Necromancer class. There's also been a lot of changes to AOE DOT skills like Caltrops and Endless Hail.

The classes and skill lines went through a bunch of changes in the interim and it would be hard to break them all down. If you can be more specific about what classes and/or content you want to hear more about, we can be more specific.