User interfaces. Very important. That's really all I want to say. But that's kind of short, so I'll keep talking.
There's more to user interfaces than where you put your toolbar options or whether your ads are on the right. Everything has a user interface, even tabletop games and movie theaters. And they all have the same basic tradeoffs to decide on.
1. Ease of use. The clearer and easier a user interface is to use, the less mistakes users will make and the easier it will be for beginners.
2. Agility of use. The faster and more complexly a user interface allows you to control a situation, the better advanced users will like it and the more skill you can allow to be involved. Interestingly, "faster" and "more complex" are deeply related, sort of like MC2 equalling E. But that's a topic for another day.
3. Use of resources. The more resources your interface uses, the less resources will be available for other things. This is perhaps the most important, mostly because nobody thinks of it. Everything has limited resources, and I don't mean RAM. I mean players only have so many fingers, so much screen space, etc. Customizing your interface to perfectly suit your system is basically optimizing your resources.
For some examples of this: most computer games use a mouse and/or keyboard input. But that's not the end of the UI. It's the beginning.
A mouse has a high-fidelity response. It's naturally an extremely agile input, although it is generally considered to only have two buttons and a scroller, which is a critical limitation.
Some games use a mouse as a straight pipe. For example, a first person shooter, where the mouse controls the view. This maps to the mouse's capabilities extremely well: two axes of high fidelity. Of course, there aren't enough buttons, so FPS games also map key controls - lower fidelity but more speed and resources.
Other games like, say, a MMORPG, mostly use the mouse as a selector tool. They abstract away the interface, using the screen as an intermediary. Basically, they create a "custom keyboard" using the mouse and screen. Unfortunately, you only have one finger (two, if you count the right click) and can't really move very fast. So, in order to create a highly suitable UI, they've sacrificed speed, fidelity, and resources. Of course, their UI are perfectly suited for the game, which makes those limited resources stretch a long, long way.
Amusingly, tabletop RPGs have made the same choice. But unlike a MMORPG, they have no intermediary. They don't limit themselves to a screen or a mouse. They fill up a sheet of paper - or, if they run out, many sheets of paper - with the relevant details, and all those details can be accessed pretty quickly because of the virtually unlimited amount of visual space and the speed of paper-swapping.
Still, every GM is horrified by the amount of "paper-shuffling" that players - especially newbies - do. "What's your attack value?" "ummmm..." "Come on... it's right next to the freaking weapon you're using!" "Oh, um, the rifle?" "YES!"
This is because although showing everything is very agile, it is overwhelming and therefore the exact opposite of easy. Careful layout can minimize this problem, but most character sheets (and rulebooks) are built by experienced players for experienced players. Ease of use is less important to them than agility. Of course, when they run into a new game system with radically different rules, they revert... "ummmmm... so how many cards do I draw?" "It's right next to your edge style!"
The wiimote has enough potential to be worth it's own essay, but for now I'll simply limit myself to "It's not a fucking gimmick."
Anyhow, think about your user interfaces - tabletops, games you've programmed, hell, even LARPs. What tradeoffs did you make? How did it synergize with your system?