I don't like the term "free as in beer". Beer is good and all, but when is beer free? Beer is cheap at the grocery store and expensive at the bar. It's not something I associate with free. If it said "Free as in smiles and speech," that would make more sense, since it's common to say that smiles are free. Just a thought.
In all seriousness though, good guide. I've managed a forum, and my information is a few years out of date. I wouldn't mind having another one, but you kind of need a wide audience to really benefit. I'm surprised Invision wasn't mentioned. They have the most advanced forum platform out there, but it's not free (neither is Jelsoft's vBulletin, which was mentioned). Invision is actually way too ambitious for all but the largest forums, with all its extra features. It can have a Wiki, a Facebook-like wall for each user, a chat room, a gallery, stuff like that. You can even get an arcade page for it, with the highest-scoring user for each game getting a trophy under their mini profile (name, avatar, join date, etc.).
In this day and age, though, you need to cater to mobile users. Ars Technica itself doesn't do this yet, but all the relevant tech forums do. There are two avenues here, and I'm not sure, on the admin side, which is easier. The first is smartphone app integration, i.e. Tapatalk. You install a plugin/modification via the user control panel, and users of the Android/Windows Phone/BlackBerry/iOS app Tapatalk can use the forum in a native app. It just works and it works real good. The second is a mobile-friendly style sheet. I know WordPress does this automatically for blogs. Some of the forum platforms probably have it built in. Access the forum in a desktop/laptop web browser, it's normal. Access it in a mobile browser, it's formatted for mobile. Not as robust as the smartphone app, but no horizontal scrolling, it all just fits nicely and looks like it belongs. Even if you don't think your forum members will want to interact with your forum on the go, consider that tablets of both sizes are coming into the living room. Sitting at a desktop is starting to be replaced by laying on the couch with a smartphone or tablet. You have to ask yourself if you want to require your members to be sitting at a desk to use your forum.