Lunara covered it all pretty well, with no inaccuracies that I can spot. There are a couple of points upon which I'd like to expound:
While we do have a contract with a Marriott, that's only one of a few reasons we've stuck with this hotel. As Lunara mentioned, the hotel treats us very well; that alone is worth a lot to the convention. There's nothing worse than having the hosting hotel staff being antagonistic toward convention attendees.
More than that, though, there simply isn't a better in-hotel location in which to hold the convention in the greater Denver area. There's only one hotel of which I know that has more space than the Marriott, but it's not well suited for an event like NDK. There are also a few other drawbacks to the hotel as well. I won't enumerate those reasons here, however.
There's also the option of moving to somewhere like the Colorado Convention Center, but I'm personally dead-set against the idea. Things just don't feel the same in convention centers. It's hard for me to explain, but I think NDK would lose something intangible by moving to such a location. It just wouldn't be the same con.
We are aware of the congestion and we're actively trying to figure out ways to help relieve some of that stress. There are a few options on the table that we're currently exploring. I can't say if anything will happen this year, but we're definitely not ignoring the problem and hoping it goes away. (The attendance numbers continuing to grow makes that a rather unlikely possibility.)
Regarding the costumes being "run into or trampled on," I haven't heard any complaints along those lines. Is this something that's happened to you or someone you know? Or are you assuming that it's an inevitability that people's costumes get ruined because there are so many other people around? Based on my own observations, most people at NDK seem to give cosplayers a little extra space to avoid causing any costume damage.
And even though I haven't spoken to the other members of the board about it, I'm going to put the kibosh on the idea that people "16 and under" should have adult supervision at all times. While we strongly suggest any young children be accompanied by their parents or guardians, requiring supervision for most teenagers would be impossible to enforce, be completely unnecessary, and add far more problems than it would solve. Most attendees at NDK, regardless of their age, are very well behaved and don't get into any kind of trouble. There are a few bad apples in every large group of people but that doesn't mean the majority needs to pay the price for the behavior of a few miscreants. (We deal those people directly.) And adding a couple thousand chaperons into the mix sure wouldn't help the congestion issues you complained about earlier.
I'm sorry your experiences with younger people have put you off so much, but I think you'll find that few people at NDK share your general opinions of them. And we're certainly not going to go to such great lengths to solve problems that, frankly, haven't proven themselves to be problems at all.
The best advice I could give is the advice that Lunara already gave: if you have a problem at NDK, go to the Ops window and report it. If another attendee is giving you trouble, your property has been mistreated, or you're having any other issues, there's someone there to listen to your problem and do what they can to help solve the problem. For more immediate help, just ask anyone wearing a Staff shirt for help and they can at least direct you to the right person or place.
We can't solve all problems, but we try our best to make sure that all attendees have a good time at the convention. But sometimes that requires your help by reporting any problems as soon as you can.