Once upon a time, I heard from someone

the only difference between programming languages is the syntax

I wanted to deny it - to say that there are other fundamental aspects that truly set a language apart from others than just syntax.

But I couldn't... So, can you?

Whenever I search Google for something like "differences between programming languages", the results tend to be debates between two specific languages (I'd like something more general) - however, some of the aspects that people seemed to debate the most were:

• Object-Oriented
• Method/Operator overloading (I actually see this rather related to syntax)
• Garbage-Collection (While it seems like a good difference, for some reason it doesn't seem that "fundamental")

What important aspects other than syntax can you think of?