Non-destructive filter layers caught my eye, and has been something I've missed since moving off Windows/Photoshop to Linux. Looking around it seems like they introduced this in V3, which is great to see.

Haven't those existed in Krita forever?

That is probably the V3 feature I'd notice most in daily use. Non-destructive editing makes experimentation much less costly; without it, every “maybe this looks better” becomes another layer or file copy.