Marianne is on a roll, I think. Her emotions and imagination is stimulated by Willoughby’s manly beauty and his more than common gracefulness and the intimacy of his carrying her in his arms. I think his rescue is an overwhelming physical event as well—I think it might have been a bit overwhelming to any young women but Marianne I feel has been constant in encouraging her romantic ideas so when this happens her feelings are magnified just as when she encouraged her grief when her father died. :D

…and in his carrying her into the house with so little previous formality, there was a rapidity of thought which particularly recommended the action to her. (Chapter 9)

His willingness to carry her “with so little previous formality” I think seems brave to Marianne—he is willing to break decorum and hold her in his arms with no thought about the practical necessity of the action. If it had been Elinor, she would have been embarrassed also, realizing the intimacy of his carrying her but she would have focused on the practicality of his doing it. Who else is there to help and it is raining so it is impractical to fetch help for proprieties sake to observe or an older fatherly type (safe) man to help or even two women or a walking stick to help her limp back to the cottage. Marianne makes a rather practical, excusable impropriety into a courageous—perhaps even defiant act. Willoughby I think enjoys the opportunity! ;D

His person and air were equal to what her fancy had ever drawn for the hero of a favourite story… (Chapter 9)

I have read “his name was good” as meaning the name was respectable but since the rest of the sentence, is about favorites, “their favorite village” and “a shooting-jacket was the most becoming” perhaps his respectability is not what is on Marianne's mind. Marianne sees Willoughby as the hero of a favorite story come true so perhaps everything about him is necessarily a favorite too—she only looks to make it so. It was established earlier in the chapter that the village of Allenham reminded them of Norland interesting their imagination but it seems to me Marianne decides a shooting-jacket is the best simply because Willoughby is wearing one so perhaps it is the same with his name—it is good because he possesses it.

Further on Willoughby, the name, Wikipedia says Willoughby is also the name of two families of British aristocrats rooted in the 1300s and 1500s. On the score of it being an old noble name, probably respectable and therefore a “good” one I see no problem.

Several on-line references give the meaning of Willoughby, from the town or habitation by the willows or willow farm—old English. * A rather simplistic idea that comes to my mind is that trees (the willow is a kind) are something which Marianne seems in raptures over during the departure of Norland in Chapter 5. I think the insensibility that Marianne attributes to the trees of Norland also apply to Willoughby—No; you will continue the same; unconscious of the pleasure or the regret you occasion, and insensible of any change in those who walk under your shade.

Ask.com and BabyNamesnThings.com linked the Second Lord Willoughby as a character in Shakespeare’s The Tragedy of King Richard the Second. Unfortunately I am not versed well in Shakespeare at all so I cannot compare the Lord Willoughby in this play and to John Willoughby in S&S, such as whether Lord Willoughby is a noble character in the play. I think though it is interesting that there is a Lord Willoughby in a Shakespearian play and Queen Mab, the name Willoughby gives to his horse in also in a Shakespearian play. ;D

Willoughby is so integral to the plot of S&S I would not be surprised that his name was chosen for particular allusions, meant to be clues to the Gentle Reader of his true character and his real intentions, but which, if any of the above might be true I have no idea. ;D