We loved Yahoo! Pipes. While there are some similarities, the product has matured far beyond the original vision of Pipes. I encourage you to check it out as it is free for developers to start out and get a feel for it.
In your opinion, are there underexplored opportunities to build on top of Pipedream as a platform, or is the opportunity still mostly for existing tools to extend themselves via integrating?
Pipedream can be used to extend existing tools but if that is your understanding of the platform then there are definitely underexplored opportunities.