The changes—which include both weapon tweaks and overhauls of entire features, such as the way level scaling works—will remain in alpha "for quite some time" so that the team can make adjustments with community input, Motion Twin said.
Custom games, announced in September, are probably the biggest single addition. They let you change the loot table to ban certain weapons, choose your starting gear and apply gameplay modifiers or timer settings.
In another major change, mobs will no longer auto-scale to your level: their difficulty is fixed, which means "you'll have to be really careful to be properly equipped before getting to late levels". It'll completely change the pace of play, and encourage proper planning and caution.
Bosses no longer drop legendary items, and you'll instead get one weapon and one active skill. You can only get legendary items as random loot in the world, or if you poke your head in a challenge door that appears after you defeat a boss. Walking through these doors will trigger another boss fight, and if you kill them without taking damage they'll drop legendary gear.
Cooldown reduction has also been rethought—mutations that granted automatic reduction have been axed, and replaced with mutations that will reduce cooldown when you perform a specific action, such as killing enemies or parrying.
If you're interested, you should really browse through the full patch notes, which contain around 90 tweaks to specific skills, weapons and items. It'll fix plenty of bugs that have been flagged since the full release in August, too, and you can expect lots of graphics and UI tweaks.