Views query building internals
As it currently stands it is not possible for Drupal to run a pure MongoDB installation with Drupal. Drupal has made a lot of progress towards abstracting out database connectivity into a central model layer with Drupal 7 and Drupal 8 promises even more. However it is still a work in progress and while a majority of the functionality is available as MongoDB backend, as it stands today a few of the components remain dependant on SQL.

As most of the work to port Drupal to MongoDB has been towards scalability and performance, it is possible to have a site where public page views are 100% on MongoDB. Performance benchmarks on a MongoDB backed Drupal instance are quite impressive.