When launching a site, it's important to go through some general quality assurance tasks to ensure the site is functioning and ready to launch.

Security

All core and contrib modules updates are applied
Give the permissions page a once-over for any glaring issues (e.g., anonymous access to "Full HTML" input format or other such silly thing)
Change the user1 username to something other than "admin"
Reset the user1 password to something that is definitely secure
Delete testing accounts
Make sure that only admins can create new accounts
Pre-provision the SSL certificates
Enforce https (in the pantheon.yml file on pantheon)
Anti-spam protection added to forms
Comment and contact modules disabled if not necessary
Check header security issues: https://dri.es/headers

Pantheon Hosted

Make sure the Pantheon APC module is installed: https://www.drupal.org/project/pantheon_advanced_page_cache
Check that Varnish is working on the Pantheon Global CDN
Make sure SOLR server is connecting, if needed. (not available for basic plan)
Check the site audit tab in the site dashboard
Pre-provision the SSL certificates
Primary domain is set for redirects
Enforce https in the pantheon.yml

Configure Cloudflare
Set up an external ping to the Drupal cron URL (and then turn off the automated_cron core D8 module?)

Drupal's css and js aggregation enabled
Page cache maximum age is set to at least 15 minutes at /admin/config/development/performance
Check the headers to make sure caching is working for properly: https://dri.es/headers
Fast404 module installed (optional)
Redis for Pantheon environments installed (optional)
Front End - Page Speed, https://tools.pingdom.com
Benchmark the site
New Relic
Disable themes not in use, even parent themes

Review Drupal Status Report Page
Review watchdog entries
Review pantheon logs
Review performance in New Relic
Install the Site Audit module and check for issues (or just check the Status tab for the environment in Pantheon)

Google Tag manager has been installed and configured to track Google Analytics (404 and 403 exclusion should be removed from the default configuration so that data goes to analytics).
External SMTP service has been configured for sending emails (recommended)
Solr server is configured and working (optional)

Domains redirect to primary domain (a dashboard setting in pantheon now)
Redirects have been created (clone our Redirects Worksheet template for import)
Wildcard redirects have been added to settings.php
XMLsitemap module has been configured and a link has been created to: /sitemap.xml
Metatag module has been installed and configured
Google Console has been registered so the new site can be submitted for reindexing after launch.

/favicon.ico resolves via the favicon module or a rewrite rule in settings.php
404 Pages are themed
All large images are being resized using responsive image styles
Breakpoint indicator is disabled
Check that we're not loading unused fonts/weights
Optimize bitmap images: http://optimizilla.com, https://compressor.io
Optimize SVGs: https://jakearchibald.github.io/svgomg/