I have studied and known Spanish my whole life, and got a job at a University where I am allowed to take some free classes. Over the past three years, I have taken all the Italian classes offered, all the Arabic classes, and now I am currently enrolled in a Portuguese class.
One thing that I am beginning to find really odd is how Spanish makes extensive use of diphthongs whereas these other two Romance languages do not. Is there a historical reason for this?
Some examples off the top of my head are:
ES: No puedo ir mañana.
ES: Yo juego al béisbol.
PT: Não posso ir amanhã.
PT: Eu jogo beisebol.
IT: Non posso andare domani.
IT: Io gioco a baseball.
With الديوان, Spanish uses a diphthong, but the diphthong [ua] is already almost there, just preceded by an [i].
But in الطوب, there is just a change in the long [u] to a more informal [o]
Overall from Arabic, there doesn't appear to be any discernible reason from a Romance Language point of view as to what happens to the vowels, although if you can understand Egyptian Arabic, it looks pretty normal.