For the last 4 years, I have been working with a lot of open source software. I once had a project with a commercial software package, and I have to say ... I really missed the open source world. But why? ... Well, there are 2 reasons:
The first reason: if something goes wrong, or you don't understand why it's going that way, you just check the source code. In most of the cases, it was the developer who implemented the link to the core incorrect. But by understanding the core, you can learn a lot. How to make a call to a core function? Etc, etc, ...
The second reason: you have the communities. People who are really willing to help you, without wanting anything for it. I've been in a lot of communities, but the community of Drupal is really crazy. There are soooo many developers in the world, sharing their thoughts on the internet. Or who are just trying to help you for free.
Now, I have this nice example of an open source world. I once had a strange problem with Drush (which is a CLI tool for Drupal). By checking the code, I could understand what was going wrong. So I logged this ticket: http://drupal.org/node/933110. Now, if you check the ticket, you see I'm actually applying a patch for the problem. But what's a patch?
A patch is a piece of software designed to fix problems with, or update a computer program or its supporting data. This includes fixing security vulnerabilities and other bugs, and improving the usability or performance. Though meant to fix problems, poorly designed patches can sometimes introduce new problems (see software regressions). Patch management is the process of using a strategy and plan of what patches should be applied to which systems at a specified time.
So the developer looked at my code, and he actually applied to patch to the source tree :-). You can see it here: http://drupal.org/cvs?commit=432252. I strongly believe that this way of working is so efficient. The product gets so easily corrected and improved ... and this is a big reason why Drupal is amazingly growing these days ...