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.
We have written just such a plugin for all our client sites.
Basically the normal Wordpress user interface was just too cluttered for clients to use. It was too confusing. So we wrote Foliopress Edit Templates. Later we used Foliopress Edit Templates to add custom fields.
We've even added an end user guide now to which you can send clients to learn how to post.