Quote from _Myst
A new topic will get a water bucket, if a topic gets over a certain number of posts it gets a lava buck. An empty bucket means you have viewed the thread before and no new posts have been added since.
A diamond means you have posted in the thread.
Quote from Vekh
A lava bucket signifies a topic with over 99 replies that has new replies.
A water bucket signifies a topic with new replies.
An empty bucket signifies a topic with no new replies.
A minecart signifies a topic that has been moved to a different section.
A dot of redstone signifies a topic that has been locked.