Just what were they thinking? They take the site down for 5 days and then put it back up before populating it with the old threads. There are only three options when you visit a message board: 1) just read, 2) post a reply to an existing thread, or 3) post a new thread. The first two capture 99% of the board's activity. By putting up a blank board they left people only option 3.
What were they going to post about? If you look at coolrunning, there are actually very few new threads each day and they usually come from whatever is happening in people's lives -- "Help! My heel hurts," "I just signed up for Cherry Blossom -- who's in?" What was happening in the coolrunners' lives over the last couple of days was, "Yuck. Look at this unfamiliar design monstrosity with no posts that messed up my user name and screwed up my running log."
If the old threads were up, this would have been a few new threads where people vented. People would respond to the existing *running* threads in time and eventually the anger would be bumped off the first page. Instead, there was no option to reply to an existing thread on running and talk about running, so it was just about guaranteed that 90% of the threads on the board would be angry complaints. Brilliant.
Also, look and feel matters. You can't just shove people who are used to a new interface and expect people to be happy because it has improved functionality (this is very debatable of course). People are creatures of habit. Look at what google did with blogger. They didn't just create blogs.google.com with a completely new look and turn it on one day with a blank page and a promise to eventually migrate your old posts. They kept it the way it was and slowly migrated it to their authentication system and database.