I think what you are seeing John is one of the limitations of photography vs. the human eye. That might sound like a bit of a cop out of an explanation but bear with me.

If you look at the night photos you have an incredible range of light, from pitch darkness through to extremely bright neon. The camera is trying to make a decision as to how much light to allow in so that it keeps the pure blacks only just black, and the pure whites only just white. This is extremely tough for night shots as you really are dealing with the whole range from pitch black to pure white, and most cameras (even hugely expensive ones) can be a bit hit-and-miss when it comes to making this decision. If they go too dark it means that you get larger dark bits where all the detail has been lost and is all just black. You sometimes see this on daytime shots where you lose a bit of detail in the shadows if they become pure black. At the other end, if the camera allows in even just a bit too much light the exact opposite happens and the highlights (bright bits) get burned in and you lose the detail in them. They kind of burn out to a pure white, and you lose the intricate details. Your first photo of Bellagio is a good one to use for an example. If you look at the very top of the tower in the middle of the hotel you can just see the lights starting to burn in and lose detail. The flip side is that the shot has some lovely reflections and textures in the lake.

There are a couple of ways to play around with this. If you set the camera to under-expose (it might be called exposure compensation on one of the menus) it will darken the darks and also darken the lights. You can save some of the detail in the lights, but at the cost of losing a bit in the darks. Any settings around -0.5 EV to -1.5 EV are worth trying.

Similary, if the shot looks too dark, try going +0.5 EV to +1.5 EV or something in between. This will make the lights lighter, but also at the cost of making the darks lighter as well. A bit like the early version of Persil automatic.

There is a technique called bracketing, which used to be hugely important in the world of film, especially for night shots, where you would do a sequence of shots some darker and some lighter. The theory being that if you did 5 you would be more likely to get one that was spot on. The ability to preview with digital helps this enormously. The Nikon has a feature that I absolutely love to bits, in that when it pops the image up for a preview it will highlight with a flashing colour any areas of the shot that is burned out. It makes it easier to just pick a small correction (-0.3 EV or -0.5 EV or whatever) and re-shoot it hoping to get everything as close as possible.

I had exactly the same problem with Bellagio when I was over in Las Vegas last November so I'll try to dig out a couple of shots that might help show what all this waffle is trying to say.