Drupal 8 follows modern PHP standards for object-orientated programming which, alone, brings Drupal much closer to the PHP family. I consider this a major improvement from Drupal 7 on the basis it will allow PHP developers to work on Drupal projects easier and understand Drupal practices much faster as well as Drupal developers to work on other PHP platforms with less of a learning curve.
Another improvement I personally have found very useful is the new configuration management system. Drupal 7 used to store all its site configuration in the database meaning the only way to access it was through a database export or Features module. Drupal 8 offers a way to export and import all configuration options between environments easily using Drush. I found using the new configuration system saved me a lot of time compared to using the Features module as having to wait for the Features UI to load can be very time consuming.
When using Drupal 8 I found using the new Block system was extremely useful and time saving. With Drupal 7 we would need to create a new module per block, each module containing an administration form, block content and context configuration all to only display a single block. Now, with exportable block configuration, we simply need to create a block plugin and we can add fields to the configuration there all in the same file, This saved me tons of time and frustration.
At the current state of Drupal 8 there are still essential contributed modules that are not released, modules like Pathauto and Tokens. Without these modules basic URL path automation, which used to be a breeze in Drupal 7, is currently impossible unless you want to spend hours writing your own code.
In conclusion, I think Drupal 8 is a massive improvement from Drupal 7 and I already prefer Drupal 8 as a platform for development, although If I was asked which one I would use to build a clients website? It would have to be Drupal 7 due to the fact it has many more fully developed contributed modules which will bring the time I spend developing right down. Hopefully this will change in 2016.
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