Considerations for mobile devices
Designing and developing for mobile phones is a whole different ball-game than traditional desktop use, but with just a few considerations, the whole process can become a lot more comfortable.
A mouse is very precise tool, the accuracy is much greater than a finger. This means any element that provides interaction will likely be required to increase in size for touch use. Also, elements that are in close quarters with one another shouldn't have destructive effects - the last thing a user wants is to delete something when they meant to edit it, for example.
Most desktop users enjoy a pretty stable working environment, the light source is generally static, noise levels are minimal and they have both hands free - this is not the case for mobile users; their light source is dependent upon the weather and streetlight, they could be in the midst of noisy traffic and they could be using their device in one hand whilst carrying other items.
Clarity is key here. Subtle shades and light colours may not be visible at all in certain lighting conditions, and notification sound effects cannot always be relied upon. So maximise the contrast and ensure your users get the message in one way or another.
There is nothing worse than losing signal on your phone - it makes the device pretty much useless, especially if all the applications require an internet connection to load information. Yours should stand out from that crowd, however. By caching data locally, you can support users regardless of their connectivity.
Short on time
If people are using your application on a mobile device, it tends to be something they want to get done quickly - few people use mobile phones as their primary web browsers, for example. Because of this you should focus on a few key elements of your website or application that people want on-the-go and strip out anything that isn't necessary to optimise the speed at which those tasks can be completed.
Should you have the resources to make less important functionality available to mobiles, don't just add more options to your menus which will clutter the UI, try to appropriately tuck it away so it's accessible, but not getting in the way of the core functionality.
When building native mobile applications, and even mobile websites (as the technologies mature), you can often access a plethora of potentially sensitive data, such as the user's location (GPS) or their address book. Most operating systems will provide warnings to the user when you try to access this data, so don't just add in functionality because you can - you might put people off using the app altogether. Assess whether data is really worthwhile accessing and whether the user expects your application to use it.
It's easy to fall into the trap of building a one-size-fits-all solution, but the screen dimensions, pixel density and hue are likely to vary between different devices and manufacturers - so it's important you test your application on each individual device you plan to support.
You should also at least make adjustments to the main design so it fits in with manufacturers guidelines, as most are very clear on how apps should be designed for their platforms, for good reason too - people don't like applications that stick out like a sore thumb because of the lack of attention to their device/platform.