Applications these days are constantly moving and migrating to the Web for one very simple reason -- the Web is a widely accepted platform and easily accessible to almost anyone. This is why Web applications are some of the most significant ways to promote content across wide audiences.
Web application design at its core focuses on the functionality of the Web application that you want to make. It needs to be simple and must offer an intuitive and responsive layout. Here are some techniques you can employ for an app. If all these requirements are met, then you'll end up with a good Web application interface.
Simplicity is the key. The more content or buttons you show to your users, the more time he or she will spend in learning what each button does, and how it relates to what they want. To avoid this, start with a few basic buttons -- it's more effective.
Nowadays you can have the functionality to give your users the freedom to determine whether they are basic or advanced users. You can walk basic users through everything, while advanced users have the option to just decide what works for them.
Special Controls or Widgets:
Special controls are widgets such as calendar, date picker, automatic form fill, sign in using Facebook etc. They are there to speed up the process for the user and for them to get to the good part right away, which is your actual content. This is not only efficient for the user though, but even for you as the content creator, since you will have everything organized in one place.
Image source: http://blog.teamtreehouse.com/wp-content/uploads/2013/04/02-bootstrap-date-picker-plugin.png
While pictures and text are great ways to communicate with your users, videos and screencasts just move you up one notch. They allow your users to sit back and learn what you have to offer. This will not only answer most of their questions, but will also allow you to express easily what you are offering.
As the competition rises, so does the library of resources out there. Using existing APIs in your Web application will save you both money and time in development. Existing APIs that have been tested provide a clear picture of how everything is working. It's a very easy way to enter the market without having to start everything from scratch.
Remember to focus on your interface elements, be consistent in your designs, use email clients, try to make used of embedded videos or screencasts, and last but not the least, use APIs. Apply these techniques and you'll see the difference soon enough.