Over at Building Windows 8, Microsoft blogger Steven Sinofsky has some background information about this patch. He says Microsoft chose to release these fixes as "post-RTM updates" rather than as a service pack:
With every release of Windows we have had approximately 8-12 weeks from when we released the code to OEMs and manufacturing and when the product was available on new PCs and for retail customers. This time has historically been used to match newly developed PCs, which can include a variety of new or enhanced components, drivers, and companion software, with the final code for Windows. Because these hardware and software components are brand new, it could be the case that they uncover the need for changes and improvements to Windows in the areas of fundamentals.Here's a list of the main improvements:
We would often create dozens of changes for each OEM for these new PCs. Those changes would be deployed during manufacturing of those PCs and thus would be invisible to customers. While those changes could potentially apply to a broader range of PCs, we did not have in place the testing and certification to broadly distribute these updates. As a result, customers would have to wait until the first service pack to see these enhancements. We know many folks would spend time working to uncover these OEM enhancements in a desire to have the most up to date Windows.
During the final months of Windows 8 we challenged ourselves to create the tools and processes to be able to deliver these “post-RTM” updates sooner than a service pack. By developing better test automation and test coverage tools we are happy to say that Windows 8 will be totally up to date for all customers starting at General Availability.
Improvements:
Known issues: