In my Drupal experience I have sometimes learned more from the failures Ive been exposed to rather than the successes that I don't fully appreciate. It took thomas edison 5000 tries to invent the lightbulb... to him they weren't failures but 5000 ways how not to make a lightbulb...
So here are a few ways not to build a successful drupal website..
1 Throw developers at the code that dont fully comprehend drupal.. IE developers who put everything in the style.css file and template.php instead of coding into a custom module or put a lot of code into blocks
2 hire rockstar drupal developers who aren't in the trenches hands on with the team and code. Pointing out what to and what not to do is helpful but not when going 200+ miles an hour around a 45mph curve (Unless of course you are the rockstar driving the car - still very risky but with the right skills and tools can be done)
3 slowly deliver features making other CMS systems more attractive.... even if the client is willing to pay thousands at the same cost of your own development to implement an "Out of the box" CMS system... no matter what anyone says all CMS systems need to have design and implementation time to successfully meet the business and functional requirements... as we all know Workflow still needs a lot of work in Drupal
4 Develop without popular well documented modules (Pleasee use Forms, cck and views modules alot can be done in these)
5 allow undocumented and unplanned feature creep in on thy fly... focus on your plan and plan to focus on how to plug one module into another with your custom theme and use cases in mind.
6 USE SVN without a release process from your staging site to production already planned out with a drupal system