Two key differences:

1. "MVCC": the ability to effectively get a handle on a static copy of the entire filesystem, perform mutations to many objects, and then submit the transaction, at which point it will either commit or rollback depending on whether any of those objects have been modified.

Windows actually has this: https://en.wikipedia.org/wiki/Transactional_NTFS allows for exactly the sort of MVCC I'm talking about—and presents a very different API than the POSIX filesystem one.

2. "Object store": as in, you don't interact with the filesystem by getting writable handles to the file's underlying backing store, where processes can effectively treat a file as persistent shared memory. Instead, you ask for a floating unnamed "buffer" object, fill it up, and then submit it to the filesystem to be atomically stored; or, vice-versa, you ask to retrieve a file, and are passed a buffer handle of the representation of the file at the point in time you asked for it (presumably, for optimization's sake, backed by copy-on-write mmap'ed pages from the latest MVCC-linearized copy of the file.)

We actually have this today as well, but in an implicit and half-assed way. Files below some size threshold, in modern filesystems, don't use any FS extents for backing, but are instead stored as part of their filesystem directory-entry node. This basically makes the filesystem into an object store for those files—but without actually exposing any guarantee to userland that those files will be read/written as atomic operations.

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To be clearer, there's a problem with this fs-atop-object-store design, as I've stated it so far:

An object store doesn't support every use-case a filesystem does, and a filesystem implemented only in terms of an object-store wouldn't be good for some things because of that. It would be terribly expensive to emulate a block device on top of an object store, for example—you'd have to make each emulated block an object, and mutate blocks by transactionally adding an updated block and removing the old block. This means that it wouldn't make sense to keep mountable read-write disk images on such a filesystem; nor would it make sense to keep database backing stores there.

But both of those things are effectively things that take no advantage of existing on top of a filesystem in the first place—they do their own index-building, their own sparse-allocation, their own journaling, etc. Making the filesystem an object store just makes it obvious that for those use-cases, the filesystem itself is pure overhead.

So, along with the MVCC object-store, the other part of my hypothetical system is a "buffer store": basically like a logical-volume manager, an API that consumes physical block devices and exposes handles to (cheap) logical resizable block devices. The object store could be implemented in terms of one of those logical block devices, but otherwise would completely ignore the existence of it. The filesystem compatibility layer, on the other hand, would allow you to request that a given filesystem object be backed by a persistent buffer (newly-created logical block device) instead of an object; and then whenever you requested that object from the filesystem, you'd get an IO handle to the logical block device, instead of an IO handle to a transactionally-resubmittable copy-on-write mmap of some object.