This release brings a major overhaul to the API of MediatR. The IMediator interface is now async-only, supporting optional cancellation tokens. The 'Async' postfix was not added onto the Send/Publish method names as there are no sync overloads.
On the handler side, you no longer need to build specific request/response types for sync/async. All request/notification types are IRequest or INotification. Instead, MediatR finds the appropriate handler implementation based on IRequestHandler, IAsyncRequestHandler and ICancellableAsyncRequestHandler.
The base class for RequestHandler is gone as well, with just the interfaces remaining.
New in this release are built-in support for pipelines through behaviors, which you can find more information about in the wiki.