For those who haven't seen it, check out AquiaTV's video on Youtube Everything You Need to Know About the Top Changes in Drupal 8 by Angy Byron (Webchick). Just watched it, and I've summarised below what I think are the greatest new features in Drupal 8.
Better UI for content editors and WYSIWYG contained in core. Previews have been fixed, and you can view a preview of a view mode.
Views in core. Say no more.
Drupal no longer supports IE6, IE7 or IE8. Which is great! As long as your clients don't use any of them.
Multilingual is in core. No more messing around with installing multiple multilingual and I18n modules. For someone that spends his days building large multilingaul sites, this is pretty exciting.
Blocks are fieldable, and you can place a block more than once.
New field types in core, datetime, entity reference, link, phone, email. I'm not exactly who could manage without a link field in Drupal 7. But this is all very good.
Mobile first, all packaged themes are responsive including the admin theme, which is great for editiing on the move.
Services are included.
Twig templating is replacing php.tpl file templating. I can't say I know a great deal about this yet, but looks interesting. Variables are automatically escaped for security.
Configuration management. Another big one. Site configurations and content types are exportable/importable through YAML zip file download, and multilingual. This is great as it reduces the reliance on the Features module, and reduces the need to import databases to get the latest configurations.
Object oriented (OO) code for modules. Great for improving the organisation of code, and good for newcomers to Drupal with a software development background.
I'm sure you'll agree this is all good stuff, especially if you would normally install contrib modules for all of the above. Well done to all the contributors and roll on Drupal 8!