I've always basically understood what (+) threads mean but is there a definition for this site? Do mods watch them more closely and/or have a shorter leash for removing someone from the thread?
So, a (+) thread is one in which all participants agree to the basic premise of the thread.
If you start a normal thread, saying, "Hey, look at this cool new game mechanic!" you may get a lot of responses from people who feel the mechanic is uncool, doesn't work, will cause your game to crash and burn and your washing machine to explode.
If you start a (+) thread saying, "I think this new game mechanic is cool, I'd like to talk about using it in D&D combats!" and folks come to tell you the mechanic is uncool, doesn't work, and will cause your washing machine to explode, they are apt to be removed from the thread.
It isn't that we watch them more closely, so much as there's a specific expectation of playing nice with the general topic that is enforced.
Edit to add: By he way, it is expected that in the first post of a (+) thread, the OP calls out the fact that it is a (+) thread. Relying on the thread title proves insufficient, sometimes.