Linux doesn't do pull requests. You actually send your patches as an email to a kernel maintainer (different people accept patches for different parts of the codebase) and then they'll review it. They'll pass it up to someone above them, and they'll pass it to someone above them, and eventually it makes its way to Linus.
That's why Linus is famous for angry and aggressive emails. He's not shouting down new developers who have made a simple mistake. He shouts and swears at the people employed below him who failed to properly do quality control and code review.
If the drivers aren't upstream, a future kernel can break them. So if you update your OS, it might no longer support your GPU without installing an alternative. Nvidia are responsible for getting this fixed in a timely manner.
If the drivers are upstream, then people aren't allowed to break them when they make changes to the kernel. There should never be a kernel or OS upgrade that causes the drivers to no longer work, the people who made the breaking change are responsible for fixing it before it gets released to you.
It's a negative that it's not upstreamable. I wouldn't go so far as to say it's bad, but it's less than good. That means you'll continue to maintain a separate kernel module that's rebuilt for each kernel.
It would need to be modified to match the kernel's coding standards and whatnot. Part of what makes linux such a strong thing is that you're just not allowed to do certain things in the kernel. They won't include ZFS in the kernel because it's got its own entire vfs -- linux filesystems are only permitted to use linux's vfs, and if you need vfs changes to make it work, you modify the 'one true' vfs. They didn't include the original AMD Radeon driver because it had its own hardware abstraction layer; you're supposed to do that in user space or not at all. (the Radeon driver was changed to remove the abstraction layer, and it was later merged into the kernel)
It's not immediately clear to me how much work it will take to get the driver into the kernel. Hopefully it won't be a lot of work, but it's possible that it would require basically rewriting it from scratch. It's at least written almost entirely in C, so that's a good start.
Regardless, this is tremendously good news.