Having a nice clean design is important...Gives the impression of professionalism from the get go.

The bulk of what makes a place 'professional' is how the members/moderators act. I definitely recommend having moderators to maintain the forum.

I've looked around for several forums that I would enjoy participating in that are mature in how they deal with issues that arise. I have left several forums because of immature members who made the places un-enjoyable for me. But once you find moderators you have to be careful. Their attitudes will become the feel and flavour of the forum. If their views don't co-inside with your vision of what the forum should be, let them go.

I would definitely avoid any kind of post ranking system. Those things are just asking for problems and leads to immature responses to contrary opinions.

just my two pennies worth...their canadian so not worth lots