I'm going to repeat that, it's important.
All of life is Interface Design.
Ok, what's interface design? In broad terms, when you try to get someone to understand how to do something, you're doing interface design. Everytime you're managing the way that someone interacts with a system, that's interface design. It's a pretty broad category, which allows me to make sweeping statements like the two up above.
How do you tell good interface design from bad? How easy is your interface to use? Ideally you want people to not even notice the interface is there. In this sense, a banana has better design than a pineapple.
Let's bring this down into the specifics. Supposing you've got a new game to play. If you've never played it before, and if you've got nobody there to teach you, what do you do? You read the rules. It's always an odious task, and oftentimes they don't seem to go out of their way to make it easier.
About two weeks ago I was helping some people learn to play the game Killer Bunnies It's a decent game, so far as I can tell. You have a certain amount of resources, you're trying to deprive your opponents of their resources, and it's complicated by the fact that you need to plan out your actions two turns in advance. I really can't give you any idea of the nuances of the game, I didn't pick up on them because I spent the entire game trying to figure out how to play cards. That's not where you want your player to be.
The game comes with a "your first game" booklet. Ok, that's good. I've been considering writing that sort of thing up for our games. It goes through some basic setup, and tells us that there are five kinds of cards; Run, Special, Very Special, Kaballa Dollas, and Play Immediately. Then it instructs you to deal with the latter two types of cards. Then it helpfully goes into how to play a bunny, which is an important type of resource in the game. It gets you through the first turn, the first round and then... nothing.
Hey wait a minute, how do I play "special" and "very special" cards? How do I play weapons? I've got a lot of weapons in my hand, and I've got no idea how to get them into the playing field. I get how you play bunnies, but most of my cards aren't bunnies, and I don't know what to do with them. From that point the game, whcih was ostensibly started, lurched from confusion to confusion until I had read enough of the second rules booklet (with helpful things like instructions for playing special cards), that finally I had an idea of what I was doing. By then the game was pretty much over. Briefly, this is the paragraph they should have put into the rules:
"Most of the time you'll play cards by sending them through the run. The run, in case you've forgotten, is that set of two cards in front of you. Cards that are the type "Run" (which include bunnies, weapons and some other stuff) can only be played by sending them through the run. You set them down in the "replace" part of the turn (Remember the basic order; flip, slide, draw, replace.), and then wait to slide and eventually flip them on subsequent turns. Special cards can also be played through the run with two important exceptions. First, instead of flipping over the top card of the run you can play a special card from your hand. Or, if you already sent it through the run and flipped it, but do not wish to use it right away, you can set it aside and "store" it for now. On your future turns you can play that card without having to forgo your flip. Very special cards are much the same as special cards, but you can play them on anyone's turn, and you don't have to forgo any flips to do it."
Or something of that nature. If that was for one of my games I'd spend quite a bit more time wrangling over exact phrasing and such, but you get the general idea. If they had that explanation in the rules the game would have been quite a bit less frustrating.
Still, that rulebook wasn't the worst I've seen. You want truly horrendous? Power Grid. Given a group of several intelligent, experienced gamers, we had to read through that rulebook at least three times before we knew what was going on, and even so we spent the majority of the first two games putting things on hold to make sure we're doing things right. It's a very fun game, with all sorts of interesting strategy decisions, but getting there was pretty agonizing.
Does any game do it very well? I don't know. I'm inclined to give some credit to Magic the Gathering, because I know they spent some time and money researching the best way to teach people how to play the game. (Fun fact: the card Giant Octopus got reprinted in 7th, 8th and 9th edition because it made for a really great picture in the "how to play" comic books. Apparently the octopus trying to eat your opponent makes it easy to remember you're attacking them, not their creatures.) Even so, I learned Magic the only really good way to learn a game I've yet found.
I got someone who knows how to play the game to show me.