Do you know the old joke that the current "MTV generation" has the attention span of a goldfish?
Well, it's not a joke. It's absolutely and completely true. (Of course there are always exceptions, but we are talking about averages and majorities.)
When talking eg. about mobile phone games, and people browsing them (ie. installing a big bunch of free games and trying them to see if they like them) it has been estimated that a free game has about three seconds to interest the player, or else the player will move to something else and probably never come back.
That's right. Three seconds. That's not a typo. Not three minutes. Not thirty seconds.
The casual gaming industry, especially on mobile phone platforms, is basically a fast food restaurant of video games. No, even "fast food restaurant" is way too slow. There even isn't any appropriate metaphor for this. People basically "channel-surf" through hundreds of free games, and if you can't get their attention in a few seconds, they will just skip your game.
One consequence of this is a very important and crucial rule of thumb for casual games: People do not read text.
The more textual information you put into your game, the worse. That's because, and if I didn't make myself clear enough, people do not read text. (Perhaps the only exception to this is if actual money is involved, but when we are talking about everything else in your game, it's a fact.)
That's not to say that your game shouldn't have any text at all. "Reading text" means "reading and understanding". In other words, any text that appears in your game must be inconsequential with respect to the gameplay itself. You of course can have inconsequential things like character and place names etc, but important or useful information should never be given to the player as text. Because, and I emphasize once again: People do not read text.
For example, adding a tutorial to your game that's mostly text-based is completely useless work. Even if the tutorial is gameplay-based (rather than just walls of text), if things are explained in text, your users will not read the text. If your tutorial eg. goes through a stage in the game and explains in text what different things do, it will be basically exactly the same as if the text wasn't there at all. If you design your tutorial so that reading the text is required to understand the tutorial, you are doing it wrong.
This can be highly annoying. In a game project I have been working with we have such a tutorial because it's the most fluent (and inexpensive) way of explaining the different actions you can do in the game and the different GUI elements. It would be quite difficult to do it otherwise. I have tried to make it as visual and as clear as possible, with very short messages (two or three short lines at most), bouncing arrows pointing to the GUI elements being explained, and so on.
But it's all in vain, because people don't read text. User testing has shown that the average player does indeed not read any of the text and gets frustrated if they can't quickly skip through the tutorial.
Currently the tutorial has simply been relegated as a button half-hidden in a settings menu, most probably never to be seen but by a very small minority of players (and from those even a lesser amount will actually go through it).
When you are designing a casual game for mobile phones, you have to assume that your average user has the mental capacity of a 3-year-old who can't read and who can't concentrate on anything "boring" for more then three seconds.
That's the mental capacity of the average user.
Sometimes I despair about the future of humanity.