Is it possible to build a high-traffic Drupal community website without hacking? I would like to find out from anyone that has built a high traffic Drupal community website ( a website with at least 500,000 visitors a month, with anywhere from 10,000 to 20,000 authenticated users at any one time ), is it is possible to create this kind of site without having to do a high level of hacking to make Drupal scale?  Can standard Drupal scale without a lot of query and other types of hacking / coding?  

Can I install Drupal, configure the modules I need, and then add more hardware and functionality (APC, Varnish, Memcache, etc) without having to do query hacking?  How do people deal with Drupal's high number of database queries without hacking, or is there no way around having to hack?  If you have built this type of community site with numerous authenticated user, what were the biggest hurdles and stumbling blocks when it comes to performance and scalability?  I already know about auth cache and pressflow.
Drupal can serve high-load sites without hacking. 

You need to use memcache to store Drupal caches not in db but in memory.
For Drupal 6 you need to use Pressflow core.
You need to use Drupal cache mechanism, its configuration much depends on what users do you have most - anonymous or authenticated.

For anonymous users Varnish can dramatically increase site performance.

Don't forget to configure cache for each View and Panel on you site.

Also Drupal 7 can work with MySQL's master-slave architecture.

Don't forget to use cdn for loading static content such as images, css, js and files uploaded by users.

If you will have search on your site - I advise you to use Apache Solr.

I have created several high-loaded sites and haven't hacked any query that was provided by Drupal core. In some cases only queries that were provided by views module were altered - but this is not a hack.

But don't think you can built a high-load Drupal site without deep knowledge of Drupal architecture, API and lot of useful modules.