The new list and library experience is here in SharePoint Online. It is great news for the SharePoint community (better integration with OneDrive, responsive, platform independent). For users with customisations or for companies who are not ready for the change yet, you can delay the new experience from the administration panel. There are certainly things which will improve in the new experience such as consistent navigation (you may notice that you lose the navigation configured in classic mode), custom JS overlays and custom branding. Microsoft should have some updates on those features in the coming year.
To switch back to classic mode for the entire tenancy:
Open the Office 365 admin centre and select “SharePoint” from the admin centre list.
Go to the settings option and set the list and library view to classic mode. This may take a few moments to apply.
You can override these settings in individual library settings if required.