In Drupal 6 fieldsets allowed site builders to group form items together. If a client asked for the ability to attach extra supporting data to a content type, you were stuck creating another content type and then create a relationship between both types using a node reference field. In Drupal 7 you would simply create a field collection for the supporting data.
What is the best way to link from one piece of content to another on the same site? You could hard cade URLs in the body of the text or you could use the Linkit module. The Linkit module gives you a pop window with an easy interface which allows you to search for content using an autocomplete field, then simply insert a link into a text area.
Has a client ever asked if you could reorder or change the name of a tab. Instead of displaying the view, edit and revisions tabs, the client wants to move "revisions" before “edit” and rename "view" to "current". This can all be archived by writing a custom module and implementing
hook_menu_alter, but there is a module that allows site builders to rename, reorder, disable and hide tabs. It’s called Tab Tamer.
Drupal's multi-site capability has been around for quite some time. Site builders are able to spin up multiple sites, using the same code base, by creating a folder in the sites directory that matches the host name and path. On a production server the URLs never change, but if you want to create a local development version of your multi-site setup that’s where things get complicated.