I think the main feature web apps bring to users is “sharing”. The share button is the equivelent of copy-paste on the desktop. Copy-paste was (AFAIC) the main feature GUI brought to desktop over CLI. It created a whole culture, the “copy-paste culture” [1]
The “share” option is why I use web apps -Google Docs is a great example of this. I think that web apps will be judged more and more on how easy and intuitive they make sharing. From simple embedding into web pages and blogs, to fine-grained sharing with access rights among team members.
Of course, if it’s sharable, it should be mashable too 
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[1] any related links? Couldn’t find any!