What's not so hot about Drupal?
This article and more to be found in:
The biggest drawback is that you will need to have above average computer skills and knowledge to install the software and some of the modules. There is not web based setup wizard to help you. (Although I have heard rumors that one is coming.) Or else, you will need to hire a professional to setup your website.
Another weakness I have found is the system lacks some groupware functionality. For example, there is an event module that will allow user to created times events and display them in a calendar, but the calendar is far from interactive. Also, the discussion forums are nice and can be used in an Intranet setting, but there is room for improvement.