There are other markup languages, like HTML, which means that learning one (like XML) will help a person learn others. I think anyone who is working with web sites needs to understand markup languages. (A point that was proven later in the class, when one of the students led us in a short Drupal workshop.)
Henrik mentions the LAMP stack in his blog post, which includes Linux, Apache, MySQL, and a scripting language (e.g., PHP, Perl, or Python). I know from conversations that organizations need people who understand MySQL as well as a scripting language. I also learned from my student this afternoon that originally you needed to know LAMP in order to install Drupal, so these are indeed worth knowing...as are the other coding languages and technologies that Henrik mentions.
How do you learn these technologies? Yes, you may be able to find a class to take. For those that want to learn on their own (and can be motivated to do so), there are sites like Codecademy that can help. No matter how you learn, be willing to learn through trial and error. Honestly, you will learn more from your errors than from your successes.
On Monday evening, I was walking to my car when my cell phone rang. Imagine my surprise when the person on the other end said she was from NBC News! She had been trying to contact me to ask in person to use a photo of mine on the evening news. The photo taken at Florida Gulf Coast University, that carries a Creative Commons license, had already been used by the Atlantic Wire, which is how she found out. But how did the Atlantic Wire find it? Metadata. (I'm sure of it!) [If you do not watch NCAA basketball, the you don't know that FGCU has unexpectedly advanced in a tournament and gained national attention.]
I must admit that this was not the first photo that has caught the attention of a company (NMAI, New Orleans, Lucille Ball's grave) I have a good eye, a decent camera, and a drive to add metadata to my photos in Flickr. In fact, when I'm looking for a photo to use, I turn to my Flickr account first because I know that I've done a relatively good job organizing and tagging my photos there.
Is metadata important? Yes...vitally important to our digital libraries and our DAM (digital asset management) systems. But there is nothing better to demonstrate the power of metadata than being able to say that metadata led to a photo from Flickr being on the evening news!