The biggest thing is "Melee 2.0". Some of the highlights:-You can equip your melee weapon by holding the weapon switch button.
-While holding it, you can hold the aim button to start "channeling", which uses your energy to make your melee attacks more powerful.
-There are "stance mods" that allow you to perform combos, which massively increase damage.
Here's the full update notes:
https://forums.warframe.com/index.php?/topic/209324-update-13-dark-sectors/