I think it all depends on the denomination. At my church, which is a large, city church, jeans are NEVER seen on the adults. Coming to church means getting dressed up in a suit and tie (generally) or khakis and a dress shirt, women will be found in dresses, pant suits, skirts and blouses, etc. Basic business wear prevails.
The teenagers wear jeans, but they are off in another building having a youth/children's worship service geared to their age group running at a parallel time to our three morning services. At my former home church (Pentecostal) women were not allowed to wear pants, so you would only see women in knee length dresses or skirts.
I would never think of going to chuch in shorts. I think it's a sign of irreverence to God. God is holy and demands just a little bit more respect than shorts and a tank top with flip flops as I've seen in many non-Protestant churches. I guess we Protestants just hold to tradition a lot more than some churches do.