This post is extremely late to publish, but I thought I would see publish it anyone, as someone might find it useful. Here are a list of important tickets in the WordPress 5.1 release.
Multisite
Introduce shared wp_blogmeta database table
Improve site creation in multisite
Implement `wp_initialize_site()` and wp_uninitialize_site()
Introduce `ms-site.php` and `ms-network.php` files
Use total users in WP_User_Query in WP_MS_Sites_List_Table
Users
Add pre filter in count user function
New filter to short circuit WP_User_Query results
Options
Add support for an optional `$options` parameter to `populate_options()`
Ensure a network’s `notoptions` cache is an array to reduce lookup overhead
Meta api
Add LIKE support to meta_key comparisons in WP_Meta_Query
Outsource network option handling from `populate_network()` to a new `populate_network_meta()`
Introduce new `populate_site_meta()` function
Thumbnails
Add filter to has_post_thumbnail() to override default thumbnail use
Coding standards
Debug
When WP_DEBUG is true show better errors for establishing database connection
Expand WP_DEBUG_LOG and make WP_DEBUG_DISPLAY work as expected
WSODs protection returns incorrect content type for JSON Requests