Salesforce puts a lot of effort into its desktop- and mobile-based UIs. These declarative UIs and related UI builder tools are the first to see advancements, such as the ability to dynamically display content based on expressions. These standard UIs and tools are also the primary method by which your subscribers will customize your application's appearance. Furthermore, the UI will evolve with Salesforce's frequent updates. In some cases, new features are available to your users even without you updating your application.
In general, aspects of your application that had already leveraged the standard UI in Salesforce Classic will just work fine in Lightning Experience without change, and will also automatically adopt the new look and feel. However, if you have utilized Visualforce extensively and/or used unsupported features...