The changelog is available here. The most notable thing is the reduced amount of SMP data transfer. There are places where more than 70% of the bandwidth could be saved! That doesn’t mean that you will not get disconnections, in particular for very complex worlds, but it’s much less likely. Also, have a look at the new configuration variable network.updateFactor. It’s set to 10 by default, increasing it will increase the latency factory for BuildCraft update messages, and may help in case of overloaded servers.
Now that this is out, I’ll take some time to finish the reviews of the worlds submitted for the world contest. Enjoy!