ClearCase novelty was in its versioned file system, which can be mounted as a virtual file system through a view, selecting a consistent set of versions and allowing for the production of derived objects. The view allows thus to map to a Software Configuration. This was a departure from the repository/sandbox model, allowing for the early management of artifacts (before they are being checked in, and not limited to these first order configuration items).
An other originality of ClearCase is its MultiSite extension. In this model, there is no real master database, but instead peer replicas, kept in-step in an asynchronous way (with close to no penalty to end-users).
In the recent years, a UCM extension has been developed, supporting a more traditional model.