Opponent Mana Visibility
I play on the Android app, and during the game, it is almost impossible to see how many lands my opponent has. Their avatar covers up the number of untapped lands if any others have been tapped, I guess because they get pushed to the left. And there's no way to click on them to get them to pop up, like tapping any other stack of cards. This impacts decision making, and in several instances, I have mis-estimated how much they had available and made major misplays that likely cost me ranked games. Like thinking they didn't have enough to buff a blocker with its ability because it looked like they only had two untapped. If lands could just be bumped down a bit or the number indicator moved to the bottom of the stack, that would be an easy fix.