Topic: Runtime feature support matrix/table
Given that each runtime has its own nuances and cannot be used 100% interchangeably (e.g. judging from the example page, the HTML4 and HTML5* runtimes don't seem to support adding multiple files to the queue), I think it's crucial that the API documentation features a feature comparison table.
Moreover, the JS API should expose a number of properties reflecting the same information (i.e. which features are available for the initiated control), so that the UI can dynamically reflect the capabilities (e.g. displaying different user instructions depending on whether single or multiple file selection is available, etc).
* CORRECTION: HTML5 does support multiple files but the dialog suggests singular - "File upload", whereas HTML4 displays the same dialog title but doesn't support multiple file selection. Can the HTML5 dialog title be altered (e.g. to "Select file(s) to upload") to make this clearer?