My biggest fear is the fingerprint sensor being janky. From what I've seen so far, early impressions are that it isn't great; especially since it was touted that it could be used with wet or oily fingers. Also, I'm iffy on the pill cutout. It looks like an eyesore that ruins an otherwise gorgeous, bright screen.
Next, the reception and call quality. I currently have a Nexus 6P, but I did end up getting a V30 last spring, thinking I'd be set because I loved the quad DAC. Nope, the radios were terrible, and LG's UI is hideous, not to mention their pittance of a theme store, when compared to Samsung.
Lastly, I'm concerned battery life won't live up to the hype of its capacity; especially in standby mode. I've seen an early battery test with the Exynos variant where it came out on top with 7:30 of SOT from simulated "real world use", but since I'm in the US, the SD 855 is it. This would be my first Samsung phone since the S3, which I hated, so the fact that I'm willing to try it again, I have high expectations.