Many features that have been discussed as desirable in MQTT, or assist with its mission to be a lightweight pub/sub messaging transport are available in MQTT-SN, along with it being a binding of a very similar protocol to MQTT on a connectionless transport.

MQTT-SN provides the following on top of standard MQTT features;
Negative ACKs on publishes and subscribes
Server initiated pings
Server initiated disconnects
Ability to update will message data at any point
Topic Registration saves space when sending/receiving on the same topic and allows subscription identification of non wildcard subs.
Will as part of session data
Client controlled timed expiry of session data
(along with other features specifically aimed at sensor networks)

A lot of the above are compelling for the current implementation of MQTT over a connected transport, and essential if MQTT is looking to extend its usefulness in mesh/sensor networks.