Drupal is an impressive system. It's surprisingly small for all it does, and it's module system is extremely powerful. But as Eli said, a lot of your tasks are going to rely on other developers doing something in a particular way.
There's a debate within the Drupal community over it's design. Drupal was around before PHP's OOP features were strong, but now that they are, there's frequent discussion about changing the system to use object-oriented data structures. Depending on your tastes, this could be a downside to you as a developer. I'm of two minds about it myself.
The system can also seem to be very "magical" to newcomers, in that somehow it does all this crazy stuff with little explanation. "I only just defined a function, how the deuce does Drupal know how to call it?!"
However, I must say that in general I'm a big fan of Drupal. It's a good system that gets loads better with each major version. I for one can't wait for 7.
