The mobile market, in both the developed and developing world is growing at a phenomenal rate, with e-commerce gradually becoming more dominant on the mobile frontier, as opposed to being a desktop/laptop-only domain. With many businesses deciding to make their customer-engagement model more mobile-centric, it is critical that we as developers get a strong Mobile UX strategy in place, as a frustrating experience on a smaller-screen platform, would surely test the more impatient users.
Another unfortunate (or fortunate depending on your views) byproduct of this mobile expansion, is the number of different platforms you have to target in 2013-onwards. Where-as in 2007 we could probably assume iPhone would be the only targeted device, now Android has taken a massive chunk (in fact overtaken Apple according to most sources) of the market, with Microsoft's Windows Mobile, and the resurgence of Blackberry's new platform, means it would be irresponsible for us to be device-biased.
Responsiveness and Customer-Driven Development (CDD) leads to a strong future for your product, ensures the targeted devices have an application that suites the operating system's design principles, to allow users to anticipate how to navigate, intuitive and consistently with the consensus of applications on that device. iPhone users are used to a stacked Table View for instance, Android has now seen the popularity of side-swipping menus appear, so users are starting to learn and anticipate buttons and navigation in certain ways and places.
Trends and flavours change rapidly over time, and so if you are creating a Web Application, you should also update and evolve based on these trends, as users who use native apps, will expect a similar sort of behaviour when they see a mobile-optimised web app.
It is surprising how ubiquitous websites focus on just iPhone, when the whole point of having a web app is to allow users from all platforms to use a mobile-screen optimised site. If it's too hard to cater for more than one platform, make the UX design neutral, no skeumorphic or menu-behaviours that are characteristics of one platform over another. The best websites are those that have been planned from the beginning, with device-agnosticism as a high-priority.
Learn from your users
I reviewed a book recently, called Lean Analytics, which would be a great resource to start understanding how to gather analytics, which will help you understand how users are using your website. Perhaps a certain UX design element is confusing, based on tracking users and the paths they take in navigating the website. Having your head on in analysing your users gives you a competitive edge in building succinct-ion in your design.
Prototyping your website, along with gathering analysis during your testing phase will help fine-tune your masterpiece, allowing you to shadow track your users to see how they see elements on the screen, what they perceive intuitively as the logical path to get to their goal. Hence, building responsive websites is an iterative process that requires constant refinement based on user responses.
Cater for future screens
The idea of having a web app that can tackle any future design trends is your goal, but starting with building an app that has fluid dimensions, catering for all screen sizes is a start. Moving away from proprietary and into open-source technologies, catering for media that scales well across screen sizes, and indeed works across all mobile platforms is part of what is called future responsiveness, working on the lowest common-denominator device.