Knowledge of Core Functionality
Drupal has numerous brilliant heads behind its amazing architecture who work hard to constantly improve Drupal's core performance. And having read in several places that Drupal's inability to optimize performance is because its core is "under the hood", it makes me wonder wouldn't it be highly unlikely that Drupal was the right choice of framework if that was the case?
As for the "functionality being hidden from developers", this feature is something which I gladly accept as "something to look up to" in high level frameworks such as Drupal. Having all the feature rich sets ( which are available to the developers by viewing the source ) well isolated and documented is is one of Drupal's principal strengths.