Technologies under the hood
And now for the real technical parts - the Geek-savvy framework-depending changes.
The PHP logic under the hood has seen drastic changes: TYPO3 7.4 is the first Open Source Content Management System which uses the newly introduced PSR-7
standard, which allows for a proper Request/Response logic known from other standalone PHP frameworks. All entry points have been unified to use a clean Application class, so that no PHP code is called in the global scope. The TYPO3 Backend now only uses typo3/index.php for calling anything (AJAX Requests, Backend Modules, etc) with a flexible Request Handling API.
All resources which were previously placed somewhere inside the typo3/ folder have been unified to be put in the according system extension. This marks a huge step towards working with other components in the PHP world. Additionally as a best practice for all extensions shipped with the core have the same folder structure.