[undees]

ClearCase is certainly popular, but I don't think it ever had a feature that was a big milestone in version control history. Client side software with a view of the source is already covered in the author's point #4, and ClearCase doesn't do this better than the others (quite the contrary, in my experience).

[ghshephard]

Clearcase had two new additions: Branching/merging made easy(ier?) and the source code under version control was seen as a file system, a drive letter on your windows system.