As the title suggests, I've got two things I wanna talk about.
Spellcaster Ability Scores
In 1E, the majority of the 9th level spellcasters had only their spellcasting stat to determine all of their spell abilities and all of their class features. This wasn't universal, as Clerics had their channeling based off of Charisma (but, this could be easily not be focused on as in-combat healing was difficult to make viable in 1E and it was often much easier to make use of low-level healing spell wands), Druids typically wanted some physical stats if they wanted to do any combat in Wildshape, and the Arcanist, Psychic, and Shaman had even more significant splits between the ability that dictated their spellcasting and the ability that affected many of their class abilities.
Generally, I think that having pure spellcasters have 1 primary stat for their spellcasting and 1 secondary stat for their other class abilities is better for class design and balance in 2E.
This change will by itself allow for 3 different ways to build a spellcaster in 2E: focusing on your spellcasting stat makes your spells powerful but limits your other abilities, which means once your highest-level spells are spent it will be time for other party members to shine. Focusing on your secondary stat (by which I realistically mean having your secondary stat 2 points below or equal to your primary stat) will allow you to make great use of your other class abilities (and perhaps do well in combat too) at the cost of needing to likely select spells that aren't super focused on their save DCs. The third and most likely option is that you keep your spellcasting stat as your highest, your secondary stat as your second-highest, and everything else after that.
A big part of this is that it would aid in class balance and avoid the situation in 1E where a primary spellcaster only needed to significantly boost 1 ability score to be effective, while other classes (combatant classes specifically) needed to focus on several stats to be effective.
It would also help make primary spellcasters be more well-rounded characters as they can make use of the other benefits of their secondary ability scores while being benefited, not punished, for doing so. It would lastly help in the situations (although rare) when a primary spellcaster would be out of spells for the day or find that the spells available to them aren't useful at all for the current situation.
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Multiple-Action Spellcasting
The thing that most excites me though is the recent revelation that metamagic effects increase the casting action requirement instead of the spell level. In general, I think this is a great change, as in 1E there were many cases where a spell with metamagic like Reach or Elemental Spell simply wasn't worth it at all compared to spells of higher levels. This issue was in some ways sidestepped by metamagic rods allowing you to spend gold to get around the spell level increase.
But, I think the new system opens up a lot of new potentials, two of which I want to focus on:
* Multi-round Spells
What would happen if you want to apply metamagic to a spell that already has a 3-action cast time? We don't know for sure. The 1E method would be to disallow using metamagic on such a spell, but that seems pretty limiting. Indeed, I think that allowing a spellcaster to "make up" the remaining actions on their following turn would be a much better solution. Now to be sure, a spell is going to have to be pretty dang good if it spends actions from two of your turns. But that just opens up the possibility of a >3 action spell+metamagic combo to be something really amazing!
I especially like the lore and flavor ramifications of this otherwise mechanical design choice. Having a powerful spellcaster spend more than 1 round to cast a super-spell would be both terrifying and exciting for the players on the receiving end. It both gives them a chance but forces them into a race against time, knowing that once that spell is completed, it's likely going to be absolutely devastating.
This would also be a powerful option if you could catch your enemy unawares, and use some of your surprise round actions to begin casting a powerful spell.
* Multi-caster Spells
Another option this allows for could be splitting the casting actions for a powerful spell over multiple spellcasters. This could either be an inherent part of being a spellcaster, or require some form of feat to perform. A spellcaster that is able to cast spells of the same level could spend some of his actions to help cast an ally's spell faster.
This could allow for some fun teamwork abilities for player spellcasters (which they are sorely in need of; the options allowing spellcasters to work together at all in any viable way were few and far between), helping an ally cast a powerful spell if you only need 2 of your actions this turn to cast your spell.
But I'm mostly excited about the options this opens up to NPC and enemy spellcasters. We'd finally have mechanical support for (outside of rituals) a group of evil cultists all chanting to create a powerful magical effect. A group of lower-level spellcasters, individually not much of a threat, could work together to cast a metamagic-boosted spell that would actually be a potential threat to the players. And so on.