The most common, most demotivating, most un-attractive mistake is not understanding how phonetically different French is from English. You simply cannot pronounce French words “the way they look” in English. Nor can you use the same intonation that you use in English. When you mix these two, you come out sounding like “une vache espagnole” (a Spanish cow) as the French would say. When they really want to patronize you, they will accuse you of having a “cute accent.”
I know a lot about this because like most French speakers, I learned to speak in high school from an American French teacher. She loved France and could teach grammar and rote memorization of vocabulary, but she spoke with a “cute accent.” My B.A. in French was Proust, Baudelaire and Balzac with more practice of bad phonetics habits.
It wasn’t until I spent a summer at Middlebury College and devoted an hour a day to training in their phonetics lab that I finally relearned French the way it really it should sound. I finally learned to say the famous French “u” and the importance of the “accent tonique.” Learning how to speak also motivated me to speak more.
I recommend to all beginners that they invest the time in phonetics training and lab work to understand the real “French accent.” It’s harder to find teachers and classes to do this, but every hour in the lab imitating how real French speakers speak really pays off in your enjoyment of the language.
You can upvote it here to add some endorphins to my day: (19) Paul Banas’s answer to What common French language mistakes do non-native French speakers make? – Quora