On the rare cases that we've rejected something for a reason the webmaster actually could fix--say, all of the links are broken--we usually do tell them. In nearly every case in which I reject a site, however, it's due to a lack of unique and original content. (Either because the information on the site is copied from elsewhere, the URL is a mirror or deeplink of another of the webmaster's URLs which we have already listed, the site is an affiliate seller, doorway or lead generator without unique products of its own, or some other violation of our submission guidelines; or just because the site is under construction, the forum has two posts in it, and there's only one line of text.)
If that's the case, as it nearly always is, then the ONLY thing the webmaster could do to "correct" the problem is to add unique and original content. Since that's the same advice I would give to a person WITHOUT any website at ALL yet, there wouldn't be much point in saying so. We used to do more of this in this forum, in fact, and all it really did was invite a lot of arguing about how unique the color scheme on mirror #451 is, how great the one sentence of text is, or why we're fools not to list affiliate sites and doorway and lead generators. The fact is, it's stated very clearly in our submission guidelines what kind of sites we are looking for and which kinds we are not. Anyone who reads them will already know why we have declined to list their site. Talking about it in more depth than that seems to create more ill will than good, frankly.