The following is an excerpt from an SMProxy packet log, showing the new 0x2C packet: https://gist.github.com/SirCmpwn/a82ce86db20b86a177ae

This packet is sent with every entity, including entities that have no custom properties sent. This adds approximately 100 bytes per entity to the network load.

The functionality of this packet could easily be provided by the existing entity metadata* system via packet 0x28. Additionally, default values should be known to the client so these things needn't be sent at all.

*The term "entity metadata" was coined during reverse engineering to describe the information being delivered over packet 0x28 and may not reflect what Mojang uses internally.

This ticket was created on the advice of Grum, via #mcdevs on irc.freenode.net.