given that there are already several websites with the set of characteristics that are needed to make you procrastinate, having one more or one less won't make a difference.

I completely agree with this. For example, before I was spending my time reading LW, I was spending my time reading something else. So while it is true that I use LW mostly to procrastinate, it does not mean that LW made my time use somehow less efficient. (Depending on whether my translation of Sequences can be considered a useful work, LW probably made me a bit more efficient. However, I would prefer to see some increased efficiency also in LW-unrelated stuff.) If I'm going to spend my time reading the web instead of doing something else, I can at least read higher-quality stuff.

As a discussion board, I would probably recommend using PHPBB. It is easy to install, and it has a lot of configuration options: using an admin menu you can create new subforums in seconds, make them read-only, or only for members, etc. (It does not have the voting mechanism like Reddit / LessWrong. However, with Reddit / LessWrong software even trivial changes are extremely costly.) Just try PHPBB, and in worst case, you will later delete it and try something else.