We've stayed at Pop Century a number of times without any real issues. The turbo-flush toilets are every bit as noisy in moderate and deluxe resorts as in the value resorts. Some noise from inconsiderate guests in the corridors and the occsional slamming door, but most people are reasonably considerate of other guests. Of course, I have been known to sleep through an earthquake or two!
We actually had more noise on one visit at YC (CL Level) than we have experienced at Pop Century.

Adults were allowing their children to run and yell in the corridors. Thankfully, we're pretty tolerant of excited little ones, and the noise tended to be after 7 a.m. and before midnight! *LOL*
At PC, we usually request a room on an upper (prefer top, but have stayed on 2nd and 3rd floors with no issues) floor, about mid-point between the elevators and the stairs, away from the pools. My favourite location is the Hour Glass Lake side of the 50s building. My BFF's favourite location is somewhere at the back of the property overlooking the parking lot. From the upper floors, you primarily see green vegetation from the trees unless you deliberately look down. It is a bit quieter there, although I don't have issues with limited noise closer to Centennial Hall--as long as I'm not near a pool! *LOL*
I believe the mattresses at PC are (or were--our last visit was in 2007) firmer than the Deluxe resorts. I tend to prefer a fairly firm mattress, while my BFF prefers more of a pillow top style. But she didn't have difficulty sleeping at PC, as long as she brought her own pillow!
Edited to add:
They had the same scratchy, cotton sheets at the deluxe, moderate and value resorts as of December 2007. Unfortunately, they use flat sheets on the mattress, which creates a messy bed for anyone who is a restless sleeper. The deluxe resorts triple-sheeted the beds, which was much more comfortable. I believe they have upgraded the sheets at the deluxe resorts since then, but don't quote me on that.
The draperies at PC were fine--they hung properly and closed completely. If a little light might bother your wife, I would recommend taking a few clothes pins or binder clips to keep them in place. I haven't had any issues with the draperies not closing properly at WDW other than when staying at Club Lake Villas (Disney Convention Center) way back in 1981!
I don't recall any light being visible beneath the door at PC or CSR (only moderate resort we've stayed at). I'm thinking there may have been a space under the door at YC, BC and BWI when we've stayed at those resorts. They have interior corridors, which may make the difference.
Question kind of off the subject but, this is my first time staying at POP...Can you see any of the fireworks from Illuminations or Wishes from here?
Wishes, no. Illuminations, some of the higher fireworks can be seen from some areas. We watched them from the Hour Glass Lake are of the 50s a few times. There may be other vantage points at PC, but we haven't actively sought them.