Answer the following question: Given the following passage  "The interface of Windows 8 has been the subject of mixed reaction. Bright wrote that its system of hot corners and edge swiping "wasn't very obvious" due to the lack of instructions provided by the operating system on the functions accessed through the user interface, even by the video tutorial added on the RTM release (which only instructed users to point at corners of the screen or swipe from its sides). Despite this "stumbling block", Bright said that Windows 8's interface worked well in some places, but began to feel incoherent when switching between the "Metro" and desktop environments, sometimes through inconsistent means. Tom Warren of The Verge wrote that the new interface was "as stunning as it is surprising", contributing to an "incredibly personal" experience once it is customized by the user, but had a steep learning curve, and was awkward to use with a keyboard and mouse. He noted that while forcing all users to use the new touch-oriented interface was a risky move for Microsoft as a whole, it was necessary in order to push development of apps for the Windows Store. Others, such as Adrian Kingsley-Hughes from ZDNet, considered the interface to be "clumsy and impractical" due to its inconsistent design (going as far as considering it "two operating systems unceremoniously bolted together"), and concluded that "Windows 8 wasn't born out of a need or demand; it was born out of a desire on Microsoft's part to exert its will on the PC industry and decide to shape it in a direction—touch and tablets -- that allows it to compete against, and remain relevant in the face of Apple's iPad."",  answer the following question. Note that the answer is present within the text.  Question: What was the "stumbling block" that Bright mentioned?
Answer:
its system of hot corners and edge swiping "wasn't very obvious" due to the lack of instructions provided by the operating system on the functions accessed through the user interface, even by the video tutorial added on the RTM release (which only instructed users to point at corners of the screen or swipe from its sides)