One of the "late to the party" features of Drupal 8 is the ability to assign the same block to multiple locations in the theme. Drupal 7 still lacks this feature in the core, but it's easy to achieve it by using a contributed module such as MultiBlock.
Drupal.org encourages developers to host all of their Drupal-related Open Source code directly on Drupal.org. This is great, because all your OS code will be associated with your account and you will get a lot of extra features such as issue tracking, release publishing, ability to reference other issues and so on. However, current interface of DrupalCore.org website is not the most user friendly, especially when you want to have a quick look at the code of a contrib module.
That's where platforms such as Github, Gitlab, BitBucket and others can be helpful.
In certain odd cases you will find yourself needing to clean a string so you can use it in a URL. Even though writing such helper function is not hard, you will soon find out that there are edge cases in which maintaining your own code does not make much sense, such as filtering out and converting non-alphanumeric characters. That's where Pathauto module comes in.
Defaults shipped with Drupal Commerce should work fine for most stores. However, exactly because of this, some of the details can be annoying and unnecessary for a specific project. The way currencies are formatted is one of those things.
In many cases the Body field in Drupal's content types can be completely unnecessary. The best examples would be nodes used for sliders, galleries, panels and more complex types where textarea field has no place.
There is a large list of helper classes that you can add to the <body> element in your theme. These classes will help you target specific contexts, nodes, user profiles, and allow you to combine these to your liking.
By default there is no way to access the language through JS in Drupal 7. You could of course read the lang attribute of the <html> tag, but that is far from being an optimal solution.
Luckily, this is pretty easy to achieve through a custom module.
Majority of our projects are still done in Drupal 7, simply because of stability and the number of available modules. Even though everyone in Drupal community is talking only about Drupal 8, it will definitely need even more time to catch up.
Meanwhile, it's finally time for me to seriously start picking up Drupal 8. This will be the fourth time: I started working with D5, shortly after moved to D6 and finally D7, with which I spent the most time. So far, the biggest change was D6 -> D7, but that is still nowhere near the number of changes D8 brings.
Feeds module gives you a very flexible and open-ended way of importing content from various sources. It belongs in the group of big and abstract modules, among with Rules and Views for example, that allow you to achieve very much, without any coding.
Sometimes the core Contextual Links module can get in your way, or be unnecessary on certain elements. Depending on your theme and workflows, you might want to disable them for certain entities. Unfortunately, the core Contextual Links module does not provide a way to disable them without custom code.