On the whole, I like self-checkouts. They're more space efficient so there's usually more of them which means less waiting, and I don't have to expose myself to the judgement of a cashier when I buy a bunch of junk food.
I know the cashier doesn't care. I care enough for both of us.
For reference, this is the kind of self-checkout I'm talking about that is now mostly ubiquitous in supermarkets:
(Image from Wikimedia Commons)
When they first came out, they had some rough edges, but I thought they'd improve over time; instead, it seems like the design has remained more or less exactly the same, which is a shame.
The Traffic Light
If you look closely at the small cap sitting on top of the cubic lamps in the image above, you'll see a small light. (It's red in the furthest left machine, and green in some of the machines in the centre.) This is the traffic light; some machines have three lights, others, like the one above, have multiple colours in a single light.
This light is only used by the assistant staff; green indicates you're having no problems, yellow indicates you will need assistance before you leave but not right now (say, if you'll need to verify your age before you pay), and red indicates you need assistance before you continue scanning and are waiting. Sometimes the light will stay green even if no-one is using the machine, sometimes it won't.
This is a good idea, but limiting it to the staff is a mistake. During particularly busy periods, I've seen a staff member tied up doing nothing but directing people to available machines since from the end of the aisle of machines, with people and their partners or trollies blocking the view, it's impossible to tell which machines are available.
I think there's a simple solution; add a blue light state to indicate that the machine is available and no-one is using it. It'd make it much easier to see which machines are available, and would free up staff to assist people rather than directing them. This is better than the light turned off, since the light is also turned off for machines that are out of order.