Target devices . If your application should run on mobile devices, your application should be lightweight on the client side (not too complicated javascript, light DOM structures ...)
Visibility . Should your application index be indexed by search engines? If so, you should remember that, for example, AJAX and Flash-based solutions will not be indexed (workarounds exist).
Usability . If your application requires high usability, you should consider creating desktop applications - in other words, Rich Internet Application (RIA)
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