Before explain myself, I want to I simply say I LOVE drupal and it is my first and usually ONLY choice when I start a new media project. I say "new media project" because you can do much more with drupal than just build websites: you can create a sms system, facebook application, social network, etc. And it makes it relatively easy for people are not ubergeeks to accomplish these new media projects.
here is my gripe - no backward compatibility (or semi b.c.) !!!!! If i am correct, drupal 5 came out about 18 mos ago, and took about 8-12 of those months for many of 4.7 modules to upgrade to 5. And now there is version 6, which is the most awesome masterpiece I have seen in a while (and that is saying alot in this web 2.0 age...) Not to mention, many of the 4.7 modules that I thought were useful never made it to 5.
Yeah yeah yeah - I know a masterpiece requires those who use them to be patient and you have to move forward and leave old things behind and "the drop is always moving": http://drupal.org/node/65922. Like I said, "yeah yeah yeah"