I've worked as a tester before, so here goes
-Running speed too slow
-Backgrounds are unnecessary and distracting, especially when mixed with the dissolving %s.
-Rotation makes it hard to navigate.
-Certain rooms, give me a Y, unbeatable on certain rotations.
-Checkpoint from north to gmay kills player if no action is taken from respawn. In general, checkpoints that make you move after respawn are annoying.
-Yen looks like a spike. More flashing?
-Pause button?! Map button?! Oh, esc. Put one it on enter and P too.
-Terminals in wrong orientations unreadable! Nearby rotators turn these into nice puzzles, though, so nothing bad.
-Rooms seem to have a lot of useless space at times, for example south. Testing would explain this, though.
-Souvenirs freeze controls, but not movement, potentially even killing you.
-Map does not follow rotation.
-Partial side wraps (Direction square check, ESPECIALLY east) confusing.
-Moving platforms outrunning player is irritating.
-In fact, conveyors in the tunnel of love are unoutrunnable too.
-Death should freeze everything, ala Mad Carew.
-If I try to move by doing a single tap in the air, it often fails to work/lags a bit. Does the movement limit run on a "modulo height" system, or "recharge" system?
-It appears that if I start holding left/right in the air, I start as slow and get faster later. That's unpleasant too.
-If I run holding left/right, then flip without letting go, I will only move once in the start.
-Fullscreen just adds black to the sides. The game itself won't increase in size.
Rotation puzzles are nice. The mechanic is fun when kept to one room, problematic when it effects others.
Rotation-Inversionplane puzzles are fun too. The fact that the inversionplane changes it's attiribute when you rotate is a nice touch.
360ntroller works fine. Beep plays.
On the other hand, rooms like Give me a Y can be used to to prevent players from proceeding on a certain rotation. However, in these cases make a rotator easily accessible, not behind a deadly room like North.
So, from these we get todos and level design guidelines:
*Place checkpoints in corners in areas where it's possible to use both rotations.
*Freeze everything on souvenir gain.
*Investigate falling dynamics.
*Fix hold-flip bug.
*Don't use partial side wraps.
*Have rotators near terminals if it's possible to be off rotation.
*And the GMAY tip above this.
The rest is up to preference and design plans, I guess.