French syntax structure is mostly in line with the other romance languages, with the very notable exception of not being pro-drop.
Edit: Vocabulary matters though because certain words are more likely to be replaced by loan words than others. More importantly, they allow linking with a language's close relatives (most importantly the other languages d'oïl), which might be more conservative, and with its historical forms across time. Said differently: languages without apparent similarity to other languages, no close existing relatives, and no written records to trace their development are isolates by definition.
Edit: Vocabulary matters though because certain words are more likely to be replaced by loan words than others. More importantly, they allow linking with a language's close relatives (most importantly the other languages d'oïl), which might be more conservative, and with its historical forms across time. Said differently: languages without apparent similarity to other languages, no close existing relatives, and no written records to trace their development are isolates by definition.