I like a dedicated Exit button and maybe Pause, but that's it.
Real coin mechs add to the nostalgia, but I also like having the option of using a button or a combo to add a coin.
It's been many versions of MAME ago, but last time I messed around with save states you were required to press a different key afterward to choose the slot.
Something about the way it worked prevented me from adding it to my cab.
I would like to get the emulated Neo Geo card option in MAME working incorporated into my cab though.
Personally, I don't like using combos.
If they are used for things like add coin, you'll have to keep reminding guests what they are.
If they're only used for things guests shouldn't get into, I guarantee you that someone will repeatedly accidentally activate them even though there is no reason for them to be moving the joystick while pressing P1 Start.