BYOND is pretty unique. I've written code in a number of languages. I wouldn't say I'm an expert in any of them. The ones I've used most lately are Python on Raspberry Pi and C++ on Arduino, both for small electronics projects. For me, it is most similar to Python, but BYOND is its own beast for a number of reasons.

• It uses syntactically significant whitespace, ie: forced indentation (like Python).
• Forward declaration of functions is unnecessary.
  This makes it unlike both Python and C/C++. In fact, most languages require that you declare a function before you call it. PHP is one of the few languages that come to my mind right away that shares this feature.
• Variable type declarations happen internally, which is more like Javascript or Python than C/C++. This gives you the ability change variable types on the fly. In fact, another nice thing is letting BYOND take care of whether your variable is an int, long, float, etc... and it usually does a great job of typecasting for you behind the scenes.
• Pretty intuitive and unique directory-tree style object oriented programming and inheritance. The inheritance system in BYOND allows you to look at any datum and understand what it inherits without having to track back through a dozen or more classes. This might be BYOND's most defining feature.

After programming in VBA, Python, Javascript, PHP, and C++, I'd have to say DM is the most enjoyable language to work in for projects that rely heavily on object-oriented programming.