All of what they showed looks really, really good. The new atlas system perfectly addressed some of the main issues in poe1: being incentivized to run the same layout repeatedly, the linear nature of progression that feels the same each league. In fact, a pretty important change here is that waypoints are detached from layouts, meaning you can always run a desired map even if you're not sustaining high tier waypoints. That means they can make map sustain meaningful once again, unlike the situation in POE1 where ever since Conquerors of the Atlas map sustain has been completely trivial.

Also very interested in the new ascendancies. Infernalist for the Witch has Pyromantic Pact, which transforms mana into a new resource that fills as you cast skills, instead of draining, but deals the sum of your life and ES as damage when you fill your infernal flame reserve. I don't know if this will work with Mind over Matter, but if it does it opens up some interesting possibilities for builds that have 0 life investment and simply coast on infernal flame taking the hits. If it doesn't work with MOM you can still do some interesting things such as using barrier invocation or cast when damage taken (if it exists) to trigger spells upon filling your infernal flame, which could cost enough mana to trigger themselves again and start a loop.

The Blood Mage also seems strong and more straightforward. One skill gives 1% increased crit bonus per 20 life, and another lets your ES from body armor also grant life. This seems like a pretty strong interaction with anything that gives guaranteed crits (I believe they showed a support gem that does this), so you could attempt to build into nothing but defence and still have respectable damage from so much crit bonus. Could be a good ignite build, since ailments are calculated based on hit damage instead of base damage like POE1 so crit probably increases the ignite damage.

Chronomancer, Chaos Monk, Gemling Merc, etc all also look very cool, can't think of any crazy good build ideas yet.

Sanctum and Ultimatum (TOTA later according to Q&A? hype!) for ascending is awesome. I can't wait to rage at ultimatum screwing me as I try to ascend for the 4th time. Also can't wait to do sanctum in maps again with a character specialized for it in POE2 combat.