I love the roof being closed when it's warm out. After tailgating for hours in the heat, it is awesome inside the stadium. I would close it 75 or above. I don't mind it staying open for cold weather, just bundle up, but I won't gripe if it is closed and comfortable. The Bear game was a lot of fun because it was actually pretty cold that day and it was raining a tad inside the stadium, I enjoyed it.
If we had a permanently closed stadium or an open air stadium, it wouldln't matter that much to me either, I would just be prepared for the temps on gameday. It's really just an amenity. I don't understand why people get so up in arms about it. Not to be lost in all of this is the stadium's design is just not conducive to airflow, therefore it needs to be little cooler then what you would otherwise consider pleasant weather to compensate.
The whole point of having a retractable roof is like having a convertible car. You take the roof down when it's nice outside and very pleasant weather. It's enjoyable.