With 5.1 out the door, our focus will shift more towards new 5.x based user interfaces while also improving the 5.x stack further, in no particular order:
HTTP cache for logged in users, detailed performance tracking, use Symfony 2.3 Proxy services, further profile & improve kernel
Symfony based login, full Bundle based permission integration for easier re-use in custom Bundles, ..
Future improved database handler and File storage to cross the 1 million content object soft barrier
New stack configuration, improved database and file storage that can scale across servers and datacenters, Async event handling with background daemon
Solr support, REST Oauth support (Symfony Bundle), WorkFlow, Subscription/Notification, Import/Export
Front end editing, UGC, language switcher, AdminBundle, EditorialBundle & Setup functionality
Tags, Page (ezflow) write support, Matrix, Identifier, XmlText html5 (editing and REST), Improved Media/Binary field types for cloud storage/CDN needs
|Extensibility||Criterion, SignalSlots, REST server, Limitations, code generation, ..||5.2/5.future|
Content (Search, ..), Comments (using content model), InfoCollector (using Symfony Forms), RSS, Staging, tip a friend, Rating
For technical info on feature statues in feature naming and comparison between 5.x compared to and 4.x, see this community maintained Google spreadsheet.