Disclaimer: I’m aware of the fact that these are not the only Romance languages in existence. Despite that, I thought it’d be much more helpful to focus uniquely on some of the most studied ones, namely Italian, Spanish, French, Portuguese and, to a lesser degree, Romanian.
Italian, Spanish and Portuguese have a more phonemic orthography than French. In French, numerous letters are silent and multiple words are pronounced identically despite having a different spelling
to my knowledge, French is the only (official) Romance language where personal pronouns cannot be omitted
Italian words ending in -zione correspond to Spanish -ción, French -tion Portuguese -ção
both Italian no and non translate to Spanish no, Portuguese não and French non and ne .. pas
the sound [t͡ʃ] is represented by the digraphs ci- and ce- in Italian and ch- in Spanish, e.g.
It. Cina Sp. China
in Italian the combination qu- is pronounced /kw/, whereas it’s pronounced /k/ in Spanish and French, and /kw/ and /k/ in Portuguese depending on the following vowel. Consider this:
It. quando [kw-] Fr. quand [kɑ̃] Es. cuando [kw-]
It. chi [k-] Fr. qui [k-] Es. quien [k-]
Port. quando [kw-] Port. quem [k-]
Italian and Romanian do not form their plurals by adding -s or -es at the end of a word like in the rest of the Romance languages, e.g.
It. buono, buoni Rom. bun, buni Es. bueno, buenos Fr. bon, bons Port. bom, bons
Italian is the only (official) Romance Language to have derived two distinct prepositions from the Latin preposition de, e.g.
Di: from Lat. de “of”
It. sono di Napoli Es. soy de Nápoles Fr. je suis de Naples Port. Eu sou de Nápoles
Da: from Lat. de ab > *d'a(b) > da “from”
It. vengo da Napoli Es. vengo de Nápoles Fr. je viens de Naples Port. eu venho de Nápoles
Italian, like French, has direct and indirect as well as stressed and unstressed personal pronouns
in Italian and Portuguese, unlike in Spanish and French, possessive adjectives are always* preceded by an article, e.g.
It. il mio libro (cf. Cat. el meu llibre) Port. o meu livro** Es. mi libro Fr. mon livre
* possessive adjectives and pronouns are not preceded by an article when the following noun refer to a family member, e.g. mia madre (not la mia madre).
** posessive adjectives proceeded by articles
in Italian the word perché means both why and because, whereas they change based on their function in Spanish, Portuguese and French
Italian per and French pour translate to both Spanish and Portuguese por and para
in Italian, Spanish and Portuguese, unlike in French, object personal pronouns are usually appended to infinitives and gerunds, e.g.
It. posso farlo (lo posso fare) Es. puedo hacerlo (lo puedo hacer) Port. posso faze-lo Fr. je peux le faire
in Italian, Spanish, Portuguese and French, the subjunctive mood works similarly but not identically
there are more cognates between Italian and French, and Italian and Spanish than there are between Spanish and French, e.g.
It. casa Fr. maison Es. casa
It. testa / capo Fr. tête Es. cabeza
It. mangiare Fr. manger Es. comer
in Italian, graphic accents are only limited to oxytones, that is to say words having the stress on the last syllable, e.g. papà, città, caffè, etc
Italian is the only (official) Romance language that phonetically distinguishes single and double consonants, e.g. caro/carro, copia/coppia, papa/pappa, etc. are all pronounced differently (with the exception of the double -rr- in Spanish and Portuguese)
in Italian and French two vowels next to each other undergo elision and an apostrophe is used, but not in Spanish and Portuguese*. This usually happens with the Italian definite articles lo/la/le and French le/la, e.g.
It. la + energia = l'energia Fr. la + énergie = l'énergie Es. la + energía = la energía
* Portuguese does use this type of contraction but it’s used almost exclusively in poetry, e.g. de + agua = d’agua.
Spanish and Portuguese are notable amongst Romance languages for not having specific pronouns referring to place and quantity
It. ci vado Fr. j’y vais Sp. voy Port. vou
It. bevo dell’acqua Fr. je bois de l’eau Es. bebo agua Port. bebo água
It. ne ho quattro Fr. j’en ai quatre Sp. tengo cuatro Port. tenho quatro
Italian and French use both essere/être and avere/avoir to form compound verbs, but not Modern Spanish and Portuguese, which only use haber/ter (or haver)
It. ero venuto Fr. je suis venu Es. he venido Port. tenho vindo
unlike French, Spanish and Portuguese, during its slow evolution Italian dropped the etymological initial h- (silent in all Romance language) in words which originally started with said mute consonant, e.g. Lat. honore(m)
Old It. honore > It. onore Fr. honneur Es. honor* Port. honra
* it should be noted that not all Spanish words starting with h- etymologically carried it, e.g. Lat. ovu[m] > Sp. huevo
