WordPress' main strength is as a blogging platform. Plugins offer extensions, but the underlying framework is fairly simplistic in terms of architectural paradigms.
In general, I would not recommend it for an eCommerce site, although it is possible.
I have experience with Drupal, Expression Engine, and MODx. Drupal makes my head hurt... redesigning a template resembles compiling Apache more than it does HTML and CSS and I honestly have no idea why it is so widely accepted. Clearly I haven't drunk the Drupal Kool Aid.
Expression Engine is not free for commercial use, but it has some great documentation and has an elegant framework under its hood (CodeIgniter). I disagree with how they implement some of their features, but it's a solid system.
MODx is lesser known, but it is the gold standard for customizing templates and it is by far my favorite system. The documentation is lacking, but it is hands down the easiest system to work with when it comes to assembling various PHP scripts and various HTML components, so if you're dealing with a team of developers and front-end designers, MODx makes it easy to integrate their work into the whole with relatively little fuss.
You can always do what the previous poster suggested: combine a couple options, catering to the strength of each.