Having fun tinkering with my picade - I replaced the artwork and put a light behind the marquee, and an additional blanking panel to stop it shining down on the screen, I also fitted a powered USB hub inside to power the light/screen from a separate psu.
But the one thing I'd improve is those two nuts and bolts that hold the control panel down.
They are fiddly the first time you fit the control panel (because you can't see what you are doing inside the case) but since then I've had the control panel off to check the speaker connections on the picade board, to change the artwork, to fix a loose joystick connection, and will be removing it again to change the buttons.
How about some sort of clamp release mechanism inside the case to release the control panel, rather than it being screwed down?
On my bartop machine I used magnets to hold down the control panel but that doesn't work too well when a over vigorous player uses it - so a clamp mechanism is preferable.
Just an idea, perhaps the hassle of refixing the control panel every time was a design decision to focus my mind on making all my changes at the same time?!
I also agree with the difficulty in changing micro SD card, however that's not something I intend to do very often.