TNB still uses a flat board, as does VNWR personally i think you are just being nostalgic
IMHO, not having any experience with forums, the flat board technique is outdated, threaded is definately the way to go or, offer both options like they do on adultwebmasters.co.uk let the end user choose what they want to use.
The only thing that a flat board has going for it is that it makes a board 'appear' more heavily trafficked than in actually is.