These answers are kinda converging. My answer is:
- What Brian Collins said—the vowel repertoire, plus the final consonants,
- and the nasals. There are nasals elsewhere, including Vulgar Latin; but the nasals are a huge part of the French stereotype. [hɔ̃.hɔ̃.hɔ̃]
- Grudgingly, I admit that the nasals are less critical than the other two. I hate both Portuguese and French because of the nasals; but noone confuses Portuguese and French.
I was reading some Old French out loud one day two decades ago, while doing my PhD. And I turned to my bolshie (literally) fellow PhD student, and commented “hey, Old French actually sounded quite nice”.
“Yeah,” he snarled. “Before it picked up all that froggie shit.”
aka: How French got all its Froggie Shit.