XBMC at it's heart is a media player. navi-x just adds a couple of capabilities to it:
1. Allows you to create "playlists" which are basically bookmarks. In the background, it processes those websites to look for playable media, then gives you a list of what it found. From there you can select a piece of media from one of these bookmarks and play it in xbmc
2. Allows you to share your bookmarks and browse other's bookmarks.
Since all of it's content is user created, it suffers from organization/UI problems.
Easy Install isn't really an addon, it's a method for installing addons. Xbmc expects you install addons from online repositories, but the chicken-and-the-egg problem is that you have to download the repository file and install it in xbmc to have access to all the addons in that particular repository. Easy Install walks you through the process of adding tuxen's hard drive (which he's opened up to the internet) just as if it were a local hard drive. Now you can use xbmc's install from zip feature to browse tuxen's hard drive and install and repos or addons he's saved there. If you have an addon's repository installed, xbmc will make sure it's always up to date so you never miss out on any new features.
There are several different kinds of addons in xbmc. 1Channel for example is a video addon. What this means is it takes the website www.1Channel.ch
and makes any content available there available to xbmc just as if that content was saved on your local hard drive.
A Program addon, such as Rom Collection Browser, are interfaces from programs outside of xbmc. Rom Collection Browser was designed to allow you to organize your ROM and emulator collection and play old school video games on your media center. The uTorrent addon, as you might imagine, lets you monitor and manage a uTorrent installation. The main benefit of this is that everything is navigated similarly and you never have to leave xbmc to do something else. It's the difference between coming across a video in your web browser and playing it on the fly, and having to download it, open a media player, then watch it. It's nothing super extraordinary, but it's a better overall experience.
As far as "What can XBMC do?," the answer is limitless. What can your computer do?