Ultimate abilities, Bottled XP, improved questing, and more

Questing

Quests are now split into acts. Completing the final quest in the act will allow you to move to the next act.
Currently there's only one act, which will culminate in a final boss of this act (which is yet to be released in a future patch). Then, act 2 will begin.
The quest list is now shaped like a tree, which should make it easier to understand the progression path through the act.
Quests can be completed and turned in even if the quest hasn't been unlocked yet - for example, if you manage to defeat a boss without unlocking its quest, then the quest will be completed anyway and you will be able to turn in that quest.

Invasion soundtracks have been replaced with more thematically fitting music.
Invasions last just a little bit longer.
Some enemies during invasions have been improved, and some have been removed completely (and will be re-added after a total rework).

The XP Bottler is a placeable object that can be purchased from the Merchant
You can interact with that device to put a talent point in a bottle
Bottled XP can be sold to the merchant for coins, as a way to make XP useful when all talents have been learned
Bottled XP can potentially be used for crafting in the future (but not yet)

DPS analyser (crafted from AI modules): can be attacked to measure your average damage-per-second. Basically an advanced training dummy.
Portable freezer (crafted from Storm essences): freezes your slime blasts, turning them into shards of frozen slime that have a much longer range and do slightly more damage.
Potion of ultimate power: instantly resets the cooldown of your ultimate abilities, but places a cooldown on all potions depending on the remaining cooldown of your ultimates

In addition to more loot, defeating a boss on Challenging difficulty will now drop a boss-specific crafting station.
These crafting stations can be used to craft all of the items that are crafted from this boss' unique drops.
Some items will have a slightly reduced crafting cost compared to crafting them at NPCs.
(note: Frostfire Pineapple's crafting station is currently useless)

Frozen orb is now considered to be an ultimate ability and shares a cooldown with other ultimate abilities.
It no longer costs mana.
The damage of the ice shards has been increased.
Added an upgrade that allows the frozen orb to be hit twice before it disappears.

Boss health was slightly increased, because the fights still felt way too short
Metal Core's rockets (in phase 2) will have a smoother launch animation, but will reach a much higher velocity over time
Storm elemental will no longer have a permanent rain of bullets on challenging difficulty, as it's challenging enough as it is
Frostfire Pineapple's dagger attack will now give the player more time to react
Slime Jackhammer's jump functionality works much more reliably, and the sound of jumping is a bit quieter
Improved the visual effect of Heal ability
Improved the visual effect of slime blast projectile explosion
Items will now be greyed out on the action bar and will not show a placement preview if you don't have the required item in your inventory.