When Drupal 8 development started, quicksketch started rallying support for and analyzing what would be needed to integrate WYSIWYG editing in Drupal core. If you haven't seen it yet, check out his excellent DrupalCon London presentation about that.
Earlier this year, Wim Leers helped lead an extensive community discussion about which editor to choose for core, culminating in Aloha editor being chosen. (For those of you who really prefer a different editor, contrib can provide those.)
We now have a core issue to add and use Aloha in core. Please help review it.
Supporting a WYSIWYG editing experience requires some additions to the Filter system. The issue for the filter system changes is currently RTBC, but more feedback on that is still welcome, particularly from anyone who maintains a filter module.