I exclusively use shift commands for admin functions, and I've never had a guest complain about the lack of admin features. I've been having parties with my mame cab for about 6 years, and with only minimal instruction guests learn how to use the basic admin features... (like game select and escape) the rest of the functions they don't NEED to know about becuase those functions don't really help enhance their excitement.
My thought is;
Real games don't have admin buttons... why should a cab? It's ok if it's a secret "shift" function... you can always tell regular guests what your secret functions are... the rest don't need to know that there is any extra admin junk.
Some of my reasoning for NOT putting various admin buttons on the CP;
1. Pause. No arcade game in the world has a pause button... why put one on YOUR cab. Plus, when I DID have a pause button, guests were always pausing games to go do stuff and then getting pissed when someone took over their game, in the meantime the machine was getting no action by anyone.
2. Enter. I use P1 for enter / game select. People remember this one pretty quick.
3. Esc. I use P1+P2 to leave a game. YMMV, but my machine only plays 1 at a time, so those keys would never get hit at the same time in real life. P1 enters a game, so it's not a big stretch for someone to remember that P1+P2 leaves a game.
4. Game list select. In the past I used the side pinball buttons on the machine to flip between lists. Most of my guests only get confused over the multiple lists unless I tell them what they mean, so most of the time I just leave the game list select feature unassigned.
5. Tab. Never, NEVER put tab on the machine unless you want to constanly fix what other people screwed up.
6. Volume. If REAL machines had volume control, it was on a pot switch located on the top, back, or bottom of the CP or front ledge of the machine. If you need volume control, put a pot switch on.
7. Coin credit. Just put a coin acceptor on (if you can afford it) Otherwise, put "Coin 1" button on the front where people might accept a coin box.
Anecdote; I have an office in my house where my business partner and I work all day. For about 2 months my friend Nate was living at my house, and in the afternoons after work he'd be in my office playing the defendercade (always Japanese nudie girl games) and we'd ask him if he wanted to run some errand or another and quite often he'd say that he wanted to finish his game so we'd leave.
One time I pressured him to come with us, and he was insistent on finishing his game... I said "dude, just pause it." He said "what do you mean" I said "just hit P1 + down on the joystick to pause" his faced dropped as he said... " you can do that" OH man, me and Barrett laughed at him. He was so crestfallen that he'd ended so many games because he'd had to leave, or go to the bathroom, or go with us on an errand.
Anyway, I can see from that why you'd want to have pause on the machine, but it's been my experience that it's much more fun to have some secrets than to give them all away. Plus you can find good times to mock your guests, and that's even more fun than playing video games.
Anyway to sum up, I've found that a shift key (secret or not) plus secret combos for enter, esc, tab, pause, list select, credit, are pretty critical.