Transferring your existing client/server or web design into a mobile application can be difficult. Mobile devices support a wide array of screen sizes and resolutions. You will also need to support both horizontal and vertical screen layouts.
The most troublesome usability issues arise when you try to implement complex, multi-step tasks that are so common with enterprise applications. For instance, the task of capturing the details of a sales order on a mobile device can be very cumbersome and time consuming as compared to using a paper-based method or traditional full-sized laptop. Implementing complex tasks such as this require a rethinking of the design and the methods used to collect the sales order data. The use of additional defaults, role-based personalization of the user interface, and extensive use of progressive disclosure techniques to shield non-important data from the user are critical for creating usable, complex mobile applications.
In addition, you need to rethink normal navigation and content display to deliver clear, concise content with minimal navigation menus, so that the limited display area can be devoted almost entirely to the content in front of the user. Of course, you must completely rethink complex desktop interaction techniques, like drag and drop and multi-windows, and address them with visual design patterns more suited for the limitations of a mobile deployment platform.