The uncanny valley is the space between being attractive and being disturbing for something non-human but human-like.
Now Jeff Atwood has written a post applying the idea of the uncanny valley to web applications that act so similar to desktop applications that they raise expectations. Jeff suggests that users might appreciate browser-based software behaving a little more like desktop software. But when the software acts 99% like desktop software, the missing 1% becomes more irritating.
I had a similar experience when I tried some of the Linux desktops around 2000. The first Linux desktops worked very much like previous Unix desktops. Then they started to look and feel more like Windows. They were just enough like Windows that they raised expectations that were soon dashed. But then instead of continuing through an uncanny valley of behaving more and more like Windows, new Linux desktops such as Ubuntu have taken a turn and have developed their own design.