I think this is a good premise - the current form of file management is definitely outdated and not nearly as functional as it could be. It seems like you guys want to do for every storage/sharing service what Dropbox does for Finder/Explorer by making management as simple and as transparent as a regular directory on your filesystem.
Here are my thoughts on this: first, I think you (or the users) are going to run into a problem eventually of redundant data. If I am sharing my photos across 3 or 4 different services, how do I keep track of the individual files? If I make a change to one, do the changes get propagated across the other services? Or do I have to do manual copies? What if I upload pictures from a mobile app or a web interface, and then upload the same files to another service? From the screenshot it seems as though you divide up functionality by individual service, which makes sens, but what if you included a 'Pictures' or 'Music' or 'Whatever' view, much like smart folders work in Finder now? And like smart folders, these views would be customizable. Would you have this integrated into spotlight or roll your own indexing feature? A good search is going to be essential for this kind of app.
I like benwerds idea of a programmable activity stream, but in order to reach a general audience with this app that kind of functionality would have to be built in. I think its safe to assume that most users aren't interested in extensive customizability and personal scripting, so even though a truly programmable file manager (possibly to replace both the command line and the file manager?) is a wonderful idea for technical users and I think something like that is in our future, but I feel like that is beyond the scope and focus of this app.

Here are a couple of specific things I would like to see:

- transparent music management. If I am browsing my music 'collection', I would like to have them appear side-by-side with my favorited songs on soundcloud, or my amazon cloud drive or spotify or youtube. When I click on the song, it will stream it from whatever source it belongs to, but will all use the same play/pause/scrub controls. This idea could also be applied to videos, or even photos and text. However of course this would mean creating your own viewers/players (so maybe not something for a 1.0) but I think a feature like that would be necessary to create the total "transparent cloud" experience, because otherwise the task of music listening becomes a chore of managing different apps, web and otherwise. "Buy/Download" buttons next to each remote song would be cool, and once bought, all that would happen is the button would disappear and the song would show up as being on your local (or on your cloud drive or whatever). It would still maintain the same position on your playlist.
Now that I look at the screenshot more, it looks like you're already doing some of this with pictures.

- Dual/split panes, a la commander. Please. It makes life so much better.

- Command line support. It doesn't need to be scriptable, but I think somehow exposing your objects to the filesystem like dropbox does would be a good solution, even though you may not be able to do it like they do by storing the files on both the local machine and the server, since I feel that would defeat the purpose of using cloud backup services. Which objects to expose and how might be a tricky question for some services (like twitter), but I just want to be able to cd around and cp files to and fro, so for services that just store/share media, this would be pretty straightforward.

- Hotkey show/hide. I love TotalFinder (http://totalfinder.binaryage.com/) and TotalTerminal for this reason. It makes general maneuvering about on my box so fluid (thats what she said), and its become a dealbreaker enough for me that I've passed up a number of "better Finders" for this reason. It doesn't have to slide in from anywhere, but I just want it to show up when I press something and hide when I press something.

More than I expected to write; I guess I've been thinking a lot about this kind of app.