Again, watch the videos and see how much of a bottleneck mechanical storage has been, it might have been a PS5 presentation but it applies equally to MS too. Both have got some pretty impressive storage solutions now and it will likely help push games on all platforms, including PC, to better take advantage of those storage speeds rather than being designed for slower storage.
Compared to the classical HHD, yes, the difference will be big (as it is now even with less than stellar SSDs), but compared to Xbox and PCs's fast SSDs, I doubt it will be significant - outside of some marginal cases (some due to bad porting, some, maybe justified from some standpoint).
I get that loading times and fast travel times will be faster (although that is still loading time), but a huge difference in gameplay... I'm skeptical. Even when used as RAM, I guess it ain't that easy or doable since the bandwidth at best (for PS5) is around DDR 3 1066MHz (8.5GB/s) and DDR 3 1333MHz, (10.6GB/s), while 3200MHz is 25.6GB. It could help to stream stuff more than a simpler HDD, but as pure RAM job... Not to mention the MTBF which would suffer a hit if the SSD would be heavily used - but indeed, is just supposition on my part.
So far the limits were mostly in terms of API, GPU, CPU, RAM and then storage (better physics, more complex and numerous NPCs, better graphics, always contributed more the the gameplay). Happy to be proven wrong, though.