Has anyone ever modified the "post-new.php" file in their WordPress installation?
I want to modify the look of this page to include pieces that I standardly include in my blog posts, and I just don't know if it is do-able/easy/worth my time.
Should I just find another CMS that allows more modifcation like Drupal?
No reason to modify the core files - you can add stuff via plugins. See, for example, the more fields plugin - it adds to the new/edit post form without breaking your ability to upgrade the core installation.
Drupal's great, but if you're focusing on blogging you may get better results tweaking WP via plugins. I use WordPress for blog sites, and Drupal for most others.