The key point to remember when developing software for any mobile platform, such as a cell phone or tablet, is that every resource is at a premium. The device is smaller, meaning:
Your user will pay less attention to your application and use it for shorter periods of time
The screen is smaller, so you can display less information on the display (don't be fooled by the high dot pitch of today's displays; reading a six-point font on a four-inch display is no fun, high pixel densities or not.)
The processor and graphics processing unit are slower
There's less RAM and less graphics memory
There's less persistent storage for your application's data
The network is slower, by as much as three orders of magnitude
Let's take a look at each of these in more detail.