Wi-Fi connectivity loss
Several Apple Support Communities threads cite a loss of Wi-Fi connectivity in iOS 6. In a massive thread titled iOS 6 WiFI Disabled, users report that the Wi-Fi option on their phone is grayed out and unselectable. Another Apple Communities thread covers a related Wi-Fi problem specific to the iPhone 5: "My iPhone 5 connects to Wi-Fi networks and remembers them, but receives absolutely zero data through the network. LTE and 3G no problem."
Numerous potential solutions are offered but there doesn't seem to be any that are universally supported. Some were able to restore their Wi-Fi access only by downgrading back to iOS 5 (which is not even possible for most users at this point). The good news is that, despite the large number of reports, most users are not having this symptom.
A footnote: Some Wi-Fi losses may stem from a problematic interaction between Wi-Fi and cellular data connections. Numerous users have found that turning off Cellular Data in Settings gets their Wi-Fi working again. Others report that data is being routed through the iPhone's cellular connection even when the device is on a Wi-Fi network. According to one theory (which I cannot confirm as accurate), this all may be a consequence of a feature, intended to be added in iOS 6, that appears to have been dropped at the last minute: Wi-Fi Plus Cellular. The problems could occur because the feature remains erroneously enabled in some iPhone 5 units or on the phones of developers who were running beta versions of iOS 6. If so, it will almost certainly require an update from Apple to fix this.
After upgrading to iOS 6, some users found that the music in the Music app was gone. This appears to happen only if iTunes Match is enabled. The precise symptom is either that music remains listed but unplayable, with all songs having a posted length of 0:00--or there is a lone Music screen that says "No Data."
One possible cause is that iTunes Match is not enabled even though you think it is. This happened to me on my iPad. I launched Settings and selected to turn on iTunes Match (you can do so either from the settings for iTunes and App Stores or for Music). While this appears to work, if you exit and return to the setting, you'll find that the iTunes Match option has reverted to Off. If your Music library is entirely based on iTunes Match, this would account for why nothing is available.
The simplest fix, if it works, is to go to Settings -> General -> Reset and select Reset Network Settings. This did the trick for me. I had to rejoin my Wi-Fi network. After that, when I next enabled iTunes Match, it stayed enabled. The Music app now showed all the music that was in my iCloud storage.
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