...can't look at the official link from Microsoft? Page 5:
Product Type|Summary of Changes
Bus Controllers|Requirements for supporting I2C and UART Bus Controllers. For implementation details see requirements under Device.BusControllers.
Fingerprint Reader|Inbox support for fingerprint readers is being introduced in Windows 8.1. In order for fingerprint readers to work properly with Windows 8.1, it is recommended to use the Windows 8.1 class driver. For more information see Device.Input.FingerPrintReader.
Graphics Adapter|
For Windows 8.1 all GPU's are required to have WDDM 1.3 driver which improve power management and performance.
Hard Drive|SATA Hybrid hard drives (as defined when the flash cache portion and the rotational component are in the same case) support is being added to Windows 8.1. For more details, see requirement under Device.Storage.Hd.Sata.
Near Field Proximity|NCI, a transport-independent communication protocol that standardizes the way a NFC controller and a device host, compliance is now required for Windows 8.1.
Precision Touchpad|Precision Touchpad is being introduced as a new product for Windows 8.1 Certification. In order for Precision Touchpads to function in Windows 8.1, they must meet the new requirements and exclusively utilized the Windows 8.1 class driver. Precisions Touchpads are required on all ARM based systems and are optional for x86/x64.
Video Playback|Video and audio playback requirements for both connected and non-connected standby systems (if implemented) to ensure glitch free video play of many common video formats. Exact requirements are under System.Client.VideoPlayback.
WLAN|Adding certification requirements to support 802.11ac, and to improve device/driver stability.
The following set of changes will be enforced after January 1, 2014:
Product Type|Summary of Changes
Audio|New communication fidelity (InAir) requirement for integrated speakers and microphone to ensure a great voice and video communication experience. See requirements under Device.Audio and System.Fundamentals.SystemAudio.
Bluetooth|In Windows 8.1, to ensure a uniform experience across Windows PCs with an integrated display with wireless capabilities, Bluetooth radio support is required if Wi-Fi is present on the PC beginning with new system submissions starting January 1, 2014.
Connected Standby Power|There are two new requirements beginning January 1, 2014. First in order to support all day usage for connected standby systems, these systems must support at least 6 hours of video playback at the displays native resolution. In addition, if these systems have a fan for a cooling, we need the fan to report its status to the Windows Operating system. Please see requirements under System.Client.PowerManagement and System.Fundamentals.PowerManagement for details.
Webcam|Starting with new system submissions on January 1, 2014, systems that have an integrated display must have a front/user facing webcam with a minimum resolution of 720p. For complete details see the requirements under the feature System.Client.Webcam.
The following requirements will be enforced after January 1, 2015:
Product Type|Summary of Changes
Trusted Platform Module (TPM) 2.0|TPM 2.0 required to support PCR [7] in system firmware. All new systems submitting after January 1, 2015 must have TPM 2.0.
See windows8-1-hardware-cert-requirements-system.pdf for all 276 pages of the 8.1 preview system specifications.