The one thing I can't recommend enough is going with the MVC (model / view / controller) approach no matter what language or platform you go with. Almost every language I can think of boasts several MVC frameworks, even Perl (the BBC came up with 'perl on rails' believe it or not).
You'll find that by using this, its much easier to introduce new features to your site without breaking the stuff that made you popular in the first place. The separation of the controller (query / logic) code from the view that is actually presented to the visitor makes things much, much easier to maintain.
The worst thing that can happen to a new site is that it becomes a victim of its own success. This typically happens when the site must scale, in a hurry, yet it cant. Addressing the technical end of that is only one part, there is also the financial aspect to consider.
For instance, SE sites are built out on a Windows platform. This means, scaling is going to mean more licensing costs, but the sites seem to be engineered well enough that by the time more resources are needed, the revenue is there. A lot of time and money went into that engineering, which you may or may not have.
As such, I recommend going with a language / framework that is portable across all platforms (Python / PHP come to mind) so that you aren't locked in. Your site will eventually take on a mind of its own and the direction it needs to go will become obvious.