Blacklight 5.0

Most noticeable distinction is the update to Bootstrap 3
Blacklight 4 was the Bootstrap 2 release
Blacklight 5 will be a bootstrap 3 release
Adding Schema.org Support
There are some behind the scenes changes that shouldn't be of too much concern to those who haven't done a lot of monkey patching.
Blacklight 5 is trying to fall back on more of the Bootstrap UI components. As an example the facets are now pure Bootstrap collapsible panels instead of rolling our own.

Upgrade to latest Blacklight 4.x and fix all deprecation warning before upgrading to Blacklight 5.
Here is a commit in Sufia that removed a lot of deprecation warnings in an app after upgrading to the latest 4.x line: https://github.com/psu-stewardship/sufia/commit/9f54aa85e6f83ecda6bd819a00b81f9f24c72321

ADA Compliance
Support for spatial data
-- Coming up with a metadata standard for place names will make creating a Blacklight extension much easier.
Book Covers Plugin
User Tagging (different flavors of tags)
Continuos Scrolling Plugin
Grid View (more views)
Hierarchical Facets (or at least improve the current pivot facets UI)
Facet Groups (admin vs. public)
Configurable open facets. Avalon has this and perhaps we can borrow some of what they have done.
( https://github.com/avalonmediasystem/avalon/blob/develop/app/controllers/catalog_controller.rb#L67
https://github.com/avalonmediasystem/avalon/blob/develop/app/helpers/blacklight/local_blacklight_helper.rb#L71-L80 )

Add your Blacklight installation to the Examples page on the Blacklight Wiki ( https://github.com/projectblacklight/blacklight/wiki/Examples )