Liked the initial need to properly manage oxygen levels, but since the upgrade didn't get more expensive, it was rather easy to get to the point where it stopped realistically mattering - especially with the ability to restore it from mining.

Would have had appreciated if upgrades showed how many tiers there are... like 1/20 or a progress bar to let player know.

Also as I wrote in comments, wouldn't have minded if there was a way to travel faster between depths and base camp, having option to build a rail and ride a minecart would have been nice even if it were a bit on the expensive side,