If you are grieving, only an agony of grief will do, and you must not let go of it.

Until you are old enough to have lost all sense of feeling- say, at 27, or 35.

Any man of virtue and intelligence(snip)

Unless, of course he is listening to your piano playing whne nobody else will.

Only rapturous delight, expressed on the proper subject, can properly be called taste.

Unless that rapturous delight that you express on the proper subject(such as one's piano playing) is balanced by behaviours that show that you have not been listening at all, in which case you have been showing "horrid insensibility".

For the proper expression of one's feelings on nature, wandering alone and possibly weeping

Except, of course, that anyone who actually has any feeling, should, like a Rousseau character, express this feeling in the most poetic terms possible, especially if the feeling is engendered by an inanimate object, such as a grove of trees, or a house. So you must verbalize these feelings in front of somebody else, not alone, and you must do it with fire and spirit and a light in your eye, which is very difficult to do if you are weeping!

But I guess whether you are wandering alone, or in comapany, and whether you are grizzling or not, expressing dissatisfaction with your lot is the height of sensibility, not merely having a teenage tantrum.

A cottage ought, properly, to have a sloping roof, snip

Actually, although this is a Marianne-like sentiment, I'm not convinced that she actually says this, or even thinks it. Could this not just be a sarky aside from the narrator?

I think Marianne has a very confused idea of what sensibility is- but then her contemporaries could not agree on it , either.