I liked Yahoo! pipes and the idea of being able to glue things together on the internet, but unfortunately many web sites simply don't have good APIs to enable this. In my experience pipes workflows were also brittle since any site could easily break them.

The lack of stable API support on (many/most) web sites (and no incentive to provide it), the likely low user/developer base, the apparent lack of killer apps, and Yahoo! itself probably all combined to prevent pipes from becoming a big thing.