Quite a few years ago I remember there was a browser Desktop based on Javascript. Critically, it was Open Source and initially quite a lot of people logged on and got involved. The main reason so many people were initially interested is because they opened and documented their API and had facilities to really easily generate applications (even a preschooler could do something). It was possible to clone an existing app for modification just by clicking a button. The source code editor was right there in the desktop too.

At first people wrote oodles of minimal apps which did totally trivial things. Many clones of the same "Hello, World" app. Then the whole thing got overrun with people putting existing Javascript games hosted elsewhere into IFrames.

Some of the problems included the fact that at the time Javascript was too slow. They also weren't able to develop the site fast enough to keep up with what people wanted to do with it. They were spending a lot of time trying to optimise client server interaction. Performance became a real issue.

The most interesting aspect of such a project is the ability to interact with other users of the desktop. But this needs to be at more than just the usual social media level. People already have social media apps in their browser and on their ordinary desktops. Putting a chat client in is not enough.

Developers need to be able to trivially add social capabilities to their apps and it needs to be possible for multiple users to work simultaneously with the same data in an interactive fashion, e.g. work collaboratively on code, drawings, documents, etc., in real time. But it can't just be a free for all. People need to be able to set up fixed groups of individuals who have access to the given data streams.

It would also be interesting to be able to easily set up scientific applications which can do distributed computing by farming out jobs to users of the system who want to contribute cycles to science.

Multiuser games also spring to mind. Basically anything where there is real time interactive content. These need not be complex to be attractive. A game where the only way to progress is to share information with other users, say via an in-game usenet system, would be just as addictive as some kind of multiuser RPG.

The one thing people go to the web for is to meet other people. Any successful Web desktop must make every feature of the Web desktop essential and collaborative. If the feature is personal and not something you want other users to be able to fiddle with then it may as well remain on one's personal desktop and does not belong "out there".

The difficulties in making such an experience seamless and smooth are substantial. Every Web Desktop out there has totally missed the point. They are closed source, so no casual developers are interested, and there's no money in it yet, so no serious developers are interested. They don't provide compelling benefits over traditional web pages and desktops and there's nothing addictive about them that gets you going back to continue your interaction with the system. They just end up being slow, boring, insecure, broken, work tools.

....Ha! That's pretty funny. I just discovered that the project I referred to above was funded by Y-combinator {turns red}.