Lots of people covered other things I dislike more, so I'll say the curved screen at edges. I liked it flat better. It also makes it so much harder to install the screen protector.
I also hated it when I had a OnePlus 8. Luckily it fell in the floor and the screen broke. My insurance repaid me the full price and with that I bought an Xperia 5 II. It is sooo much better with its small form factor, flat screen, its stereo front facing speakers, it's headphone jack and its long battery life!