Classes
Architecting the core of your game from the beginning is an often-skipped process. Because we're too eager to just get on and build our game, let's jump straight in and start placing assets in a scene, adding them as we go. This kind of practice is fine for prototypes (mostly, however, even with prototypes, a level of architecture is usually required). When building your actual product, however, without setting up a proper architecture from the beginning, you are heading toward a world of utter mess.
When we say architecture, it doesn't mean that you need to design everything (but it helps). You just need to ensure that you plan what you are going to build before you build it instead of thinking about stuff and checking Google for information on how to do it. Even if you are using some kind of an agile method, you should have a good framework and goal for each sprint as you plan for each sprint. This will guide you on what should be done and when, not just designing the project on...