Major There is no way to set properties on the will message. This may have been discussed but on implementation it seems like a significant omission and asymmetry. Content type and payload format, are two examples of properties which would be very useful.
The behaviour of properties on retained messages appears to be unspecified. It could be interpreted that properties are not stored nor propagated, but that seems unlikely to be desirable. In the event of propagation, the behaviour of some properties such as the publication expiry interval, should be clarified. I suggest that the "time of receipt" for a retained message should be the time of receipt of the subscribe request which triggers it.