After going to the Verizon store and playing around with both the Icon and the M8, here are my impressions:
Form Factor: To be honest, I'm not a huge fan of the way either looks. The Icon looks like a brick, it just has a weird shape and is a little too thick. The M8 is thinner, but I don't like the silver backing at all. Also I'm a fan of the built in Windows buttons at the bottom, as well as camera button on the side (Icon has both of these).
Operating System/Apps: Turns out Verizon isn't shipping the Icon with 8.1, which is a huge bummer. I know you can get the developer's preview for it, but I'd much rather an official download. But rumor has it that they're releasing 8.1 "soon". As far as apps goes, the Nokia apps blows HTC out of the water. The HERE maps/driving apps are awesome, while HTC doesn't offer anything that I'm interested in.
Camera: Everyone says the Icon has a better camera specs-wise, but when I was playing around with the phones, the M8 camera just seemed...clearer. I'm not going to put much weight into my observations though. According to specs, it seems like the M8 has a better front facing camera and the Icon has a better rear facing camera.
Price/Brand: The Icon is $50 cheaper, and I trust the Nokia brand more.
Miscellaneous: Icon has wireless charging, while the M8 has removable storage. M8 slightly edges it out on CPU/Battery, but Icon wins with RAM/megapixels.
I really don't know what to do! Please weigh in with your opinions. As of now I'm leaning towards the Lumia Icon for the Nokia/HERE apps alone. But if the M8 had a matte black backing, I may have picked that no questions asked!