- Native integration (such as drag'n'drop from a file manager, copy-paste of pictures from other applications, opening of local files)
- Responsiveness
- Keyboard Shortcuts
Beyond those requirements, it seems like a web browser can do almost anything required for any kind of text-mashing or abstract, logical application. There are apps like word processors which live or die based on performance of particular functions like turning fonts into pixels, but almost any communications-based application won't need to do something like that.
Will it?