Drupal 8 is the latest instantiation of the popular Drupal Content Management system. Although not an entirely new product, Drupal 8 represents a significant upgrade from 7, and users hoping to upgrade their modules and themes to 8 might suffer from a steep learning curve. For module development the code is now split basically into two halves: php5 style hooks are retained in the .module file but much of the code is now moved into class definitions using php7 OOP features. While the latter is nice I wonder why half the system using the old PHP5 syntax has been retained. Either one approach or the other is desired; by choosing both the developers have overly complicated module development to the point where it will appear unattractive to would-be new Drupal developers, and old ones will be tempted to stay where they are with 7.
One of the major problems is the fragility regarding the installing/uninstalling of modules. Drupal 7 was more forgiving in that respect. You could delete a module on disk and the module would disappear from the modules list. Now any such action, even an attempt to put back a deleted module, renders the entire Drupal instance unusable. Modules can then neither be enabled nor disabled. The only option is to reinstall everything from scratch. A similar situation arises frequently whenever some mistake is made in development and a package becomes broken. This kind of time-wasting is what turns developers off. What 8 lacks is simplicity. Power doesn't have to equate with complexity at all. Sure there are some nice new features in 8 but instead of a few mandatory files in the module folder we now have multi-way dependencies, 'routes', libraries, entities, controllers, interfaces, configurations and loads of stuff that should either be fully documented or left out. Any incorrect change to one of the example programs seems likewise to break the system. Unlike in 7 you don't seem able to alter an installed module by renaming properties and methods. There's too much copying-in of original files into 'installed' data in the database, which creates fragile dependencies. After a week and a half I'm calling it quits. It's just not worth the effort.
For those of you thinking that they will eventually migrate to 8, my hunch is that 8 will never make it to the big time. As Steve Ballmer used to say: 'Developers, developers, developers, developers ...' Wagging a big stick at them and telling them that they really shouldn't be doing X won't persuade them to bat on your side.