graeme_p - 12:29 pm on Feb 9, 2012 (gmt 0)
Be a bit careful of the example of others. I have no experiences of forums, but there is bad database designs in some reasonably popular CMSs.
I suggest you read up on data base normalisation. Also foreign keys and transactions.
Databases keep your data safe, and structure it in a logical and maintainable way. You sometimes need to compromise that for performance, but most websites do not need to, even if they need to they should try caching first (and you can cache the results of queries, as well as web pages).
Slightly OT: if you are writing a forum from scratch, are you using something like plain PHP? Web frameworks are a lot more productive.