Please look at
http://daringfireball.net/projects/markdown/ for a reference and implementation of markdown.
Reddit.com notably uses markdown instead of the square bracket tags like ar15.com and other forums do.
Also consider using names for subforums in urls instead of database keys. So instead of:
http://www.ar15.com/forums/forum.html?b=8&f=10
for the Florida hometown forum, something like:
http://www.ar15.com/hometown/florida
and instead of:
http://www.ar15.com/forums/forum.html?b=2&f=124
for the magpul industry formum, we should have:
http://www.ar15.com/industry/magpul/