Controls
To create a great player experience you have to start with the players interface with that experience. This essentially means the control mechanics. If the player is constantly struggling with the control system, and the challenge is in actually trying to control what they are doing rather than achieving goals then your very unlikely to ever achieve Flow. If the game feels good to control, and is simple to understand the more fun an experience it is.
Having done a lot of work with motion control recently, this became very obvious. The problem with motion control is that the player can very often feel they are fighting against it. There's 2 main factors in this, one is environmental effects on the controllers as in lighting conditions and colours, and the second is lag. As a designer you can not control the environment your game is going to be played in, you can recommend the ideal setup on start up and hope players will want to make the effort. Likelihood is that some players will not make the effort, and you have to accept their probably not your target audience, the same goes for people who just want to sit and slouch.
Lag is the other issue and it can vary, it just takes time to process the input from the devices and then implement that in-game with everything else that is going on. The way we got around this in The Shoot was to have the cross hairs on screen so that you are focusing on moving the cross hairs around rather than aiming down the controller, because if you did you would notice that the cross hairs do not perfectly match up to where the controller is pointing all the time.
Motion controllers are certainly an interesting challenge, another potential disconnect for a player is that there is no physical feedback, so if you're swinging a sword around in game, when you hit something the object in game may stop but your arm keeps moving as there is nothing to resist it. You then get a scenario where your arm is out of sync with the object your controlling. This is not an issue if your swinging a light sabre around and slicing through everything of course.
Something I noticed when we were developing using the Guncon was that it actually required a certain amount of skill to calibrate the controller correctly. Another challenge with motion control is letting the player know what they physically have to do, its easy saying press up on the stick or press this button, but you have to show players what they need to perform.
I have always felt that new ways of interfacing with a game would lead to new and exciting experiences, we just have to make sure they are not just contrived replacements for previous control methods.
Flow
A bit more about flow, as I think the controls are a primary factor in achieving this along with suspension of disbelief. Flow is often described as being in the zone, which is a pretty good description, what it means to me is that the experience is so enjoyable that you forget you are playing a game. You're not fighting against the controls, you're not asking yourself what happened there, if you fail you know it was because you did something wrong.
In terms of game controls it's like when you learn to drive a car. While you are learning to drive, you have to think about changing gears, using the clutch, using your indicators, checking your mirror along with actually navigating the vehicle around roads and not crashing.
When you have learned to drive all these things become instinctive, and rather than thinking about the process of making the vehicle move you concentrate on the road. The changing of gears and clutch control become something you do without thinking. When enquiring about a car people often ask what it feels like to drive, and this is directly applicable to games.
If a player picks up a controller and a game feels nice to control, it behaves how they expect it to behave and they understand what they need to do, then attaining flow will be a lot easier and you will be a large way towards creating an enjoyable experience. It doesn't really matter whether the player is using a mouse to click on things, a traditional game controller or a motion controller device, the same principle applies.
Motion control
2 other aspects to consider when designing a game using motion control devices are fatigue and reaction time. Players get tired playing the game, so short burst gameplay is probabbly the best choice. Moving an arm or your body takes longer than a quick move of the thumb, so this has to be taken into account in the game design in how quickly you expect the player to react to events.
