Text Transparency --- not just with a animations or by converting it to a picture
Cropping tools that actually are easy to use --- already better with Affinity
Smoother animations and transitions -- Keynote's transitions are so much smoother and natural feeling.
Better implementation of alignment guides --- already better with any Affinity product.
Better Text Kerning/Tracking -- already better with Affinity
Creation of Guides -- Seems so simple just to pull them out from the ruler
Pictures in boxes just SUCK in PowerPoint --- Affinity Publisher beats this hands down.
Instant Alpha on Images --- Keynote make this easy
Ability to apply image effects from the main menu instead of digging through menus
PowerPoint struggles with Large Videos.
I'm not saying that PowerPoint isn't feature rich, but Microsoft (in general) have the ability to implement features in the most obtuse way. I'm just imagining the flexibility of all the tools in the Affinity ecosystem (much like the workflow that can be achieved in Publisher) and how they are implemented is such a clean, intuitive, and powerful way. Perhaps just adding some transition and animations to Publisher would be enough.
You can certainly achieve most things in PowerPoint, but you have to have a higher skill level to do so, and often need an entire suite of applications to achieve the results. In the end this is probably not any different than an Affinity app...but there is already an established way to share features across the applications.
I'm sure I could come up with more... as could anyone else that has used any tool other than PowerPoint.
In the end Affinity tools are just easier to use than PowerPoint. It's also a space not currently occupied by Adobe.