Very impressive and ambitious UI, lots of great attention to detail. Click-and-drag selection highlighting in particular. Here's some observations about the interface whilst I was exploring. This is Chrome/Ubuntu.

On the document search page:

* Double-click to open isn't intuitive for me on a website. Had a few puzzled seconds wondering how to open a document.

* Middle-click to open is broken.

* If I try to do a drag selection starting over a thumbnail, it doesn't work.

* The browser right-click context menu isn't always suppressed, and completely obscures your custom menu. Maybe float it to the left of the cursor so both are visible?

* Seems like the thumbnail/list view switcher buttons are the wrong way round.

* If I select multiple documents then click download, nothing happens. Either grey out the option, or add zip downloading.

* Selecting 'view pages' when in thumbnail view breaks the layout.

* The 'Page x of x' navigation at the bottom is non standard without being any more useful. Found it annoying having to actually type in the page I wanted to go to. Duplicating the pagination nav at the top of the page would be nice.

* Left-hand document list is well thought out, like that the scrollwheel works, but it still feels a bit awkward. Not sure why.

* 'Entities' seems too specific to describe what are essentially tags. Caused a bit of confusion. Phone numbers and 'terms' are not entities.

* The 'Log In' button is a bit out of context next to the other buttons. Maybe move it to the top right next to your logo?

* As the document links are taking me off-site, I'd like the statusbar url to still appear on hover.

* Found myself focusing the search box just so the text was dark enough to read.

Document viewer:

* The document sidebar might be better on the left. Having it on the right breaks Fitt's law for the scrollbar.

* Arrow key and spacebar scrolling doesn't work on document load. You have to click to focus the document first.

* The document page navigation might be better in the top bar, rather than the sidebar. Having it in the sidebar gave me the false expectation of sidebar content that changed related to the page I was on.

* The 'Pages' thumbnail view only seems to show up for some documents.

* The zoom level I set for the 'Pages' view also changes the zoom level for the 'Document' view. So I can't easily browse with tiny thumbnails but fullsize documents.

* Annotation UI is great, very intuitive. The restrained use of colour on the rest of the site really pays off, that yellow is hard to miss.

* With two notes parallel to each other cause their markers to overlap, example:

http://www.documentcloud.org/documents/29627-cosa-letter-on-...

* From the 'notes' view I should be able to click on the passage in the note to get back to its original document context.