I don't really use WP or Joomla, but a rough set of pros and cons for drupal after using it for about 7 years:
- Extremely flexible, can import or manage all kinds of content and produce extremely professional sites - for instance importing a hotel feed, adding a proximity search, faceted search filters for price, star rating, facilities etc is all in a days work
- Most things you might want to do has been done before as a free module that can be bent to your will - there are a massive number of plugins available
- Designed to handle any kind of content in any way - ie you could create an accounting package with each item an invoice, your product feed site could have real products with fields + filters for tech specs etc rather than just pasting the info into a 'post' on wordpress - or you can even run it as a full ecommerce site
- can manage users and add social networking style stuff quite easily.
- Tho improving, nowhere near as intutive as something like WP, particularly for no tecchies like freelance writers etc
- *Steep* learning curve
- Most things you want to do has been provided in about 8 or 9 different modules that don't play nicely together, possibly have been abandoned and there is no clear guide to which is actively worked on and useful now, rather than years ago
- Likewise there are many ways to go about achieving the same outcome (eg building a photo gallery, or organising your content into say country and category) and it is hard to know which is the best way to go until you've done the work and tested it yourself
- The flexibility encourages developing for drupal - but they make major changes to their API with each release, generally making coding far more long winded
- Initial installation package is so stripped down you need to add a bunch of modules for every install just to get it doing pretty basic stuff that comes as standard on things like WP
- Significant lack of quality free or commercial themes and designers. Design work generally a fair bit more expensive than for WP which everyone uses and understands
- Significant lack of documentation and a history of rather rude responses to requests for help on the forums and issue trackers
I could go on under both headings. Basically it is extremely powerful, and up to a point you can plug in lots of different modules to build something quite clever completely free - but you will often find they never quite do everything you need. For most static sites its a bit like using a sledgehammer to crack a nut but if you want to build something dynamic like a web application it can offer shortcuts by being able to recycle existing code and functionality from all the free contributed modules - just be prepared to spend some time debugging.
Edit: Just to add I think if I was making sites for other people to use or if I'd started later I'd probably be well into WP - I feel a bit committed to Drupal now I know it works but I think WP has come on leaps and bounds in terms of flexibility and often wonder if my life would be simpler if I just studied how to bend WP to my will instead.