Most of my buyers eventually ask me a great question (I've been fortunate to have so many wonderful & smart clients): "Once we've completed our inspections of the home we're buying, what are appropriate & reasonable repairs to request the sellers to complete?"
My position, which I share with both sellers and buyers, is that it is reasonable to request repairs of any active water or pest damage (usually identified on the pest inspection report) and any safety related issues (faulty electrical wiring). It is also acceptable to request that structural or system defects are fixed if they were not obvious when the offer was made.
Items such as damaged roofing and faulty windows are not usually known to buyers when they make their offer. The buyer likely assumed that these items were working properly when they made their offer and the price they've agreed to pay for the home did not factor in these repairs. Thus it is reasonable to expect the sellers to repair these items, offer credits sufficient to cover the cost of repairs or reduce the sales price accordingly.
On the other hand, if the sellers disclosed that the air conditioning does not work properly, the 
buyers probably took that into consideration when making their offer. They've likely reduced the price accordingly or asked in the offer that the air conditioning be repaired. In my opinion, it is not usually appropriate later in the deal to request this repair be completed since it was known to the buyer prior to submitting the offer.
Ultimately all repairs are negotiable. It is up to each buyer to decide what they'd like to request and each seller to decide if they'll agree to all, some, or none of the requested repairs. If the buyer has not removed their property condition contingency, they can cancel the purchase if an agreement on repairs cannot be reached.
When both parties realize that no home is perfect and negotiate in good faith, there are rarely any serious issues over negotiating repairs.
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