You can buy whole enclosures if you want to really keep adding things to this computer. It's not the most elegant solution for every situation, but yes, it does provide a measure of expandability and oddly enough something that gets overlooked. The Thunderbolt ports provide a surprising amount of things you can upgrade and add to this computer.
It's a very fast machine in other typical tasks such as office type documents (iWork), and web browsing. That's why, in my view, it's not a total wash in terms of upgradability. Interestingly though OWC offers a SSD upgrade though. At the time of purchase you need to purchase the ram and SSD it comes with. The only sore point is that you can't upgrade RAM. The Retina display definitely does make a difference.
The screen is incredible and makes everything have that extra 'pop'. I ran some initial tests with XCode,iPhoto, and Aperture. In terms of processing power and performance this is dynamite.
