I'm also unable to complete this Windows 8 update. When I click on Turn Windows features on or off, the .net framework 3.5 is at the top like you said, but with a expand/collapse menu containing two items/checkboxes inside:
1) Windows Communication Foundation HTTP Activation
2) Windows Communication Foundation Non-HTTP Activation
I initially checked both and Windows 8 failed configuring these features after prompting me to restart. I THEN checked only option #1, which installed successfully.
Taking into account another user's advice, I cleared out the Windows Update cache by deleting everything C:\Windows\SoftwareDistribution\Download. I then restarted Windows Update and rebooted once prompted. The update still failed.
No idea what's going on here! Can someone please help?