I think an age restriction on bucks would be the best way to go for restricting buck harvest, but as already said, it is hard to judge the age of a deer on the hoof, and even many very experienced hunters cant do it very well. It would just be too hard to impose that kind of restriction, though I do think it would give the best results for quality deer management.
Here in wisconsin the DNR is focusing on the doe populations, at least in my neck of the woods, and we have to shoot a doe just to get our buck sticker allowing us to harvest a buck at all. That helps the deer herd in general, but wont have any profound effects on the quality of bucks. Having bigger bucks, for the time being anyway, I think will primarily fall on land owners and hunters. It's a slow process, but even in the last few years I have seen a big jump in my area of people getting on the QDM wagon. The hunting shows help promote it, and having hunters and land owners getting together, talking about it, hanging out their signs, all that stuff, really helps. Obviously the most direct and immediate effects are seen when you make something a state wide law or regulation, but for growing bigger bucks, it's a hard regulation to make.
Not only that, I don't really want the DNR telling me I can only shoot a certain age or size buck. We all hunt for different reasons and making us pass on bucks because they are not "trophy bucks" isn't fair, especially to kids. I dont want to have to tell my kids their first year or several years of hunting they wont get to shoot a buck because we rarely see large bucks on our property. now that I'm an adult and can access different areas and have more experience hunting, I am fine with passing on small bucks. but thats a choice i think we should all get to make ourselves in the woods. yeah, it sure sucks to pass on a nice 2.5 year old just to see him riding home in the neighbors truck, but that's hunting, thats what its all about. I'm all about doing whatever I can to help our area see bigger, more mature bucks, but hunting is about more than that, and if we start making too many regulations just in the name of bigger bucks, hunting will just become about shooting big bucks, and that would be a shame.