I know some would say we're just conditioned to expect a mouse and keyboard combo, or a gamepad when playing games and it just requires thinking outside the box. But there's no doubt about it, a lack of any sort of meaningful input device on mobile devices is a huge barrier in mobile game design. We're only offered a very basic, imprecise input device. This basically boils mobile game controls down to prodding or swiping with a finger or tilting the device, making it difficult to implement many of the traditional genres we know and love.
Of course it is a very different market than for web based, or traditional PC and console games, so I suppose it comes with the territory. Dumbed down, "one-button" and grid based puzzle games, with little depth, are what the devices thrive on. But I'd argue that the market is due to the limitations of the device, not the limitations of the device are due to the market. If, for instance, you want to implement a retro action game, platform game or anything at all that involves shooting (to name but a few) you're really out of luck.
The main problems with designing mobile game controls are:
Being forced to put your hand/finger right over what you're looking at and/or tilting the device obscures our view of what's going on. This is all without taking into account the greasy finger smears that can end up partially obscuring the screen.
Touch screens obviously lend themselves towards mimicking other pointer devices such as mice, trackballs and graphics tablets. The problem is they are much less accurate than all of the traditional pointer devices. This means hit areas such as buttons need to be bigger and people with large hands and fingers often struggle.
This and the visibility issue are huge inherent problems with touch screens and a big part of the reason why touch screens are also a bad idea on desktop and laptop PCs.
Lack of Multiple, Physical Buttons
Without multiple, physical buttons your control options are seriously limited. Say you're faced with having to design a game to coincide with the release of an action film. Then you're struggling. Prodding a screen with a greasy finger, whilst watching a constantly moving character simply jump through hoops just isn't going to do it.
These devices were never made for games. Their crude, simplistic input devices were designed for a totally different purpose.
If you need to implement multiple distinct interactions required by most retro game genres (such as movement, jumping, attacking/shooting) without a keyboard or controller you're left with having to use a virtual, on screen controller. Again, this can obscure the on-screen action (particularly on small smartphone screens) and provides none of the feel or physical feedback of a physical input device.
The other alternative is to try and force a basic point and click based control system into every game. This leaves you in a similar position to only having a mouse for PC games (but with much less pinpoint accuracy). It's ok for certain genres, but try using just a mouse to control a retro 2D platform game. This is why these sorts of games tend to turn into overly basic single action, one-button games, with most other actions happening automatically.
Physical Controller Add-Ons
Physical gamepad controllers for smartphones have started to emerge, usually clipping onto and extending the phone. This is a step in the right direction, but I worry about the practicalities of having to carry around a physical add-on that is at least as big as the smartphone itself. These are meant to be mobile devices and with smartphones getting bigger and bigger anyway, is anyone really going to be bothered to carry another bulky item around with them? There's also the price issue. These controllers can often be an expensive addition to an already extremely expensive device.
A Solution is Required
As it stands now, gaming on smartphones and tablets is great for the Nintendo DS casual crowd. Although these are often people that wouldn't normally play computer games. Until the control issues on mobile devices are sorted out it's going to be extremely difficult to lure gamers away from consoles, PCs and even dedicated handheld gaming devices. Anything other than simple timewaster games are difficult to pull off on mobile devices and at best involve some serious compromises, in terms of controls at least.