1) HDR+ is largely useless and does not give an accurate picture. It's akin to people who like to boost the color/contrast/sharpness because they think it looks good. Obviously taste is subjective but I don't see many being a fan (including myself)
2) ARC (audio return channel) is only important if you want to run audio to a soundbar or receiver via HDMI separately from the HDMI connection to your TV. It is limited to 2 channel PCM or 5.1 Dolby Digital/DTS (same limitation as optical).
Love HDR+ mode myself, I use it for anything that's not gaming and not hdr. Think it shines when watching sports. Can definitely see some people not liking it though.