For people that have played DND for a while, then that "old school" feeling is partly based on nostalgia.
Humans are biased as a species, and I don't mean racially, I mean in almost every decision we make. The funny thing is we often don't know it, and when we do, we often don't believe it. We are psychologically built to be confident in ourselves it would seem, to believe we are better than we are....even when science proves us wrong.
Humans tend to like things that are familiar, and as you get older more things get familiar and familiarity gets more ingrained. So as you get older, there is a biased towards things that are new and different....even when you feel you are completely unbiased.
Nostalgia can also be enhanced by another facet. For the majority I people (I am assuming), gaming gets harder as you get older. Its harder to get the old gang together, harder to play for the same amount of time you used to, and harder to focus on your character (as the demands of real life consume a lot of your thoughts). So with that in mind its easy to look back at your gaming days of old and think they were better than they are now. And I would suspect in many cases....that is completely true. Personally, I feel that gaming during my college days was more fun that it is for me in the real world. Its not that gaming isn't still fun, but real life has dimmed the shine a bit.
**Note, its important to realize that the last paragraph I wrote is biased based on my own experience. I could be completely wrong based on scientific evidence, but right now I feel very confident in my assumptions. I felt it was important to highlight this since I am writing an argument talking about biases.
Now all of that said, is nostalgia a bad thing? It depends. Since gaming is an activity solely for the sake of fun, it doesn't matter why a person does or doesn't enjoy certain things that are new or old or whatever. As long as they find something they enjoy, then its fine.
The problem comes in with new systems and new concepts, and the debate that causes. We all have our preferences and biases, whether we realize them or not. Some cling to the familiarity of old, and have an inherent bias against new things. Others often a penchant for wanting things that are new...and will think they are better than they are. These biases skew our arguments when we are comparing new and old.
Ultimately, all of this only really matters to WOTC and other gaming companies. We can debate different editions and styles of gaming on Enworld until everyone has achy joints from typing so much. But in the end we will all find something we like and stick with it. The problem is....in order to continue the legacy of DNd, new players must be found. Dnd has to be fun for these new players.
Now we veteran gamers can tell WOTC what we think is "fun", but our opinions are biased....and don't necessarily reflect what the newest generation thinks is fun. Gauging the new generation, and scientifically determining what they will enjoy, can be extrememly hard....and in that respect nostalgia is a powerful enemy.