This is the first formal Java 8 release.It contains a bunch of bug fixes both in the engine (multiplayer, inventory, etc) and in modules like Throughout the Ages.
Full engine change set: http://jenkins.terasology.org/job/TerasologyStable/54/
Yep, skipped another number (build 54 failed due to a missing config tweak in Jenkins). We need to go to a different numbering scheme for the Omega releases, but that'll still be a ways out.
New modules: AnotherWorldPlants, CheatsForAll, ComputerMonitors, MobileBlocks, ModularComputers, SimpleFarming, SimpleLiquids
More details: http://forum.terasology.org/threads/pre-alpha-release-55-and-other-news.1301/