Right now, the anti-griefing system always and by default protects any and all signs and other decorations. This wouldn't be bad if there was a possibility to turn off ALL protection. What's the point in having a separate build mode when decorations are already protected by default? Especially signs, they are very commonly used and are some of the most basic items. My idea is that it should be possible to completely disable ALL placed block protection, so that the most basic non-solid blocks can be placed in frontier cities such as Spawnville without fearing that nobody will be able to change them. A while before writing this thread, I changed a sign that was previously unprotected. And then I wanted to change a roadsign that was placed in a weird way, but that one was protected and I couldn't do anything with it. If there was a possibility to turn off block protection, and protection was disabled by default, whoever placed that roadsign would most likely place it unprotected, as roadsigns should be "public domain" blocks. //EDIT: I just found another weird side effect of anti-griefing: A torch that is standing in the air without a supporting block.
There is, you can turn this on using: /noprotectmode. After that every block you place will be fully unprotected, no matter what kind of block (common/uncommon). Note that if you run into those problems you can always use /staff (to check for online staff) and ask them for help to remove those. Edit: After re-reading the Anti-grief wiki page (force of habit; I always do that when I come across comments / criticism / possible improvements / etc.) I have to agree that this option is a bit buried. I'll see if we can put a little more attention to it.
My opinion, the first time a player places a block that's protected, it should tell them it's been protected and what command to use if they want it to be not protected, as well as a link to the wiki page that you just linked.