Shaddah

How about you tell us some unique aspects of the Arabic language?

I would definitely start with the Shadda, a linguistic term to indicate when a letter is doubled, stressed while pronounced, but only written once, with this symbol on top of it, to indicate that it's stressed.

Keep in mind, if the stress is not pronounced, it's very much likely that you will end up saying a complete different word.

For example, we have Hammam and Hammam.