Forum upgrade complete + what's ahead...

Because /forums tries to look for a file "forums.html" or "forums.php" and can find neither. /forums/ searches for "index.php" or "index.html" in the /forums/ folder, which exists.

Almost right. The server expects the trailing slash to be included (so as to indicate a directory on the server), otherwise it treats it as a file. The request for /forums will not work because you're telling the server to look for a file called forums (note: no extension) instead of a directory called forums.

EDIT: I've reconfigured the server to treat both requests as a directory request, though I would advise everyone to continue to use /forums/ as the forum's settings treat that as the real path to the forum.