Lines 21-2 Virtue itself turns vice, being misapplied,
And vice sometime's by action dignified
The whole monologue of Friar Lawrence (lines 1-30) provides an epitome of the Renaissance worldview, but these two lines are the cornerstone of their ethics, that implied a 'careful' use of boldness as well as the acknowledgement of the great rule of chance/Fortune. "Virtue" retained its original Latin meaning of both physical and inner strength. This approach will be destroyed in the 18th century by the -- devastating -- model that sets supposedly universal theoretical principles to be applied each time.
Lines 65, 69 Holy Saint Francis! // Jesu Maria
The former exclamation is maybe a joke of Shakespeare, but the latter comes from true parlance, and is still used in Southern Italy (rather than Verona, in the NE): Gesummaria!
Lines 85-7 . . . her I love now
Doth grace for grace and love for love allow;
The other did not so
paraphrasing the Gospel of John 1.16-17
Mercutio's description of the then (late Renaissance) fashionable continental style of dueling is ironic. The same kind of description, but much longer and meant as highly honorable, will be used by G. B. Marino in Adone, canto 20, stanzas 233-247.
In Mercutio's list, Dante is conspicuous by his absence. To think that he was so warmly welcomed in Verona! Paradiso 17.70-75.
The famous women exalted by poets are mocked by punning on the sound of their names: "Dido, a dowdy," etc., with the exception of Cleopatra, "a gipsy" -- that however puns on "Egyptian."