The installation differences between Joomla and Drupal are, really, negligible. My personal preference leans towards the explicitness of Joomla's install, but that's not much of a differential. More noteworthy is the option to include sample data in a Joomla installation, something that can be a big help in getting beginners started.
WordPress, however, wins hands down when it comes to installation simplicity. If that is something you need, then this is the CMS to use.
While installation of these content management systems are similar, the same cannot be said for their interfaces. Drupal, Joomla and WordPress each present their content in different ways and manage content under different paradigms, as well.
This is really the biggest gap between these three platforms, because the fundamental way in which they treat content is vastly different.
For Drupal, all content is contained within blocks, and the content's type (blog, article, product in a catalog) determines which block the content is displayed in.
Joomla lets you create content within categories too, but content is additionally tied to location on specific pages.
WordPress handles all articles as either posts in a blog or a page on the site, which is fairly intuitive for new users to grasp. Location of content and widgets is handled by dropping them within pre-set locations within a given theme.
Those are just some of the differences. Here are more.
Drupal's administration is handled by a master control bar that appears on the top of the screen whenever an administrator or super-user is logged onto the Drupal site. There isn't a "back-end" control screen, per se -- administrative pages appear as an overlay on the site, such as the Create Basic page screen.
The advantage here is that all of Drupal's controls adapt to the page you're on, so you don't have to "assign" actions to particular pages unless you're working on a site-wide activity. Indeed, the main Dashboard for a Drupal site is often bereft of features -- you can pick and choose exactly what controls and status messages can go into the Dashboard.
But this flexibility can also be a big obstacle for new users, since it's not always easy to see what's going on. The flow of Drupal's administration works nicely, once you figure it out. The learning curve for understanding how Drupal does things is sharper than for Joomla and WordPress, but the payoff is great: Overall, the administration interface in Drupal is much smoother and powerful.
The back-end of Joomla looks like what a site administration control panel should look like: A single set of segregated pages that collect all the administrative tools in one place.