Form Factor: If you prefer a non-metallic finish and physical buttons the Icon is the one.
OS/Apps: The least weight of your criteria. Both devices will (Icon eventually) have 8.1. HERE apps are available to all Windows Phone devices. The bigger issue is Verizon support. As the owner of three Lumias (920, 1020, 1520), I can state that there is no exclusive Nokia app that is a deal breaker for me.
Camera: The Icon takes the lead here. There are many reviews concerning the M8 camera, the majority are not complimentary.
Price/Brand: Icon is cheaper and supported by Microsoft. HTC has a negative record supporting Windows Phone devices.
Miscellaneous: Wireless charging is convenient but for me is not a deal breaker. For me the deal breaker is removable storage which I find indispensable in my 1520. CPU/memory goes to the M8 but there is little difference in performance due to the operating system.
Bottom line: Unless you're camera priority and comfortable with cloud storage for music and data, the M8 with a DotView case (preferably the Xbox one

)