There's a few usability quirks that immediately sprung to mind;
I didn't find it particularly intuitive to actually dive in use:
1. When you first set up account and start a workspace, the first thing it asks you to do is choose the people you want to share it with. This seems "wrong" to me. I would expect to be able to set up a workplace, add documents and wotnot and only then be bothered about who I wanted to invite in.
2. I then tried to work out how to add an item to a workspace... there were buttons on mouse over for Manage, Share and Delete - so I sat there trying to find 'Add document'. It wasn't until I eventually clicked on the workspace that I 'entered' it. I wasn't really expecting that 'enter to do something' metaphor. How about a mouse-over 'click to enter message'.
3. Add video resource - how about some clue as to the formats you accept. None that I tried. that's for sure. It's a bit distracting to see the Web page URL input go scrolling by on the way to the file chooser.
4. Thumbnails of the documents would be good.
5. The Black pallette really isn't to my taste.
6. The big one... without the ability to actually edit Word or Excel files (as opposed to annotate), it isn't really a collaborative tool in my opinion, I'll probably keep using Google Docs or Drop Box and I suspect others will too. Sorry, it's a lovely idea apart from that. I'm not sure I can even see myself using it with the free account.