I agree. I have all sorts of views on the casualness of dress at certain restaurants (and don't even get me started on how some people dress when going to the theater!). Maybe this makes me a dining elitist, but I think there is a time and place for casual dress, and as you stated Proof, PS7, Corduroy etc is not that place. Now I think dark or nice jeans and a button down shirt or cute blouse is fine for some of these places. But not your favorite t-shirt sporting the name of your college on it.
Maybe it's a more old-fashioned view point but I think it also has to do with respect for your surroundings and the food you are about to be served.
I definitely think it's a more old-fashioned view point. But, there's nothing wrong with that. Similarly, I don't see anything wrong with someone going to Proof in a t-shirt. I also see no problem if Proof wants to turn them away. In any scenario, diners simply need to weigh all these factors and decide where they want to dine. How important is it to you to be surrounded by well-dressed diners? How important is it to the 25-year-old to be able to wear a t-shirt and have a fancy meal?
And, "respect for...the food you are about to be served" is certainly a "dining elitist" type of comment. Again, nothing wrong with that. This message board is full of people who know what they are talking about. However, one must realize that 99% of the people dining out aren't the type of people on this message board. The average poster here can probably rattle off 50 DC chefs off the top of their head. The average diner might be able to come up with one...as long as that one is on Food TV or was appeared on an episode of American Idol.
The dcdining.com world is a different one than the general population. And as someone who is more like the general population, I love that about this place. My dining frequency has declined greatly the last few years. However, the quality of my dining has skyrocketed since finding this place a couple years ago. You guys are the experts; in the top 1% of the population when it comes to dining. You know the dos and don'ts. But, you have to realize most don't know and they never will. In fact, they are the ones who will change the dos and don'ts, especially in areas such as how to dress. If the restaurants you love are going to remain successful and thrive, they will be forced to change to accommodate society's changes. They can't survive only off of the business of the top 1%.