It's a good watch.
To me, some lessons are:
- for user tags to be useful in an OPAC, you need critical mass, or seed the catalog with tags from an external source (LibraryThing?)
- while there are a lot of similarities in the way the interfaces work, there are some things that *maybe* should be standardized; like the location of facets, labelling, etc. A research opportunity.
- If I had to pick an [open source] discovery layer from those presented, I'd have to say Scriblio; it just seems solid, everything looks dead simple and keeps you clicking. One thing I'd change is the facet layout, as they seem rather "mashed together"--whitespace to the rescue?