My questions is: will the new (December, 2012 release date) Imac's have similar problems?
That question assumes that the majority of 24" iMacs out there have the same issues, which is not the case. Sounds like your computer is in need of some troubleshooting and/or servicing. My input on those issue you describe:
long delays in videos loading before playing - or not even playing at all
If you're referring to online, streaming videos, then your internet connection might not be fast enough (4MBs is really not that fast for modern HD video), or, as pointed out, there is a software issue (whether it's browser-, plug-ins-, or OSX-related needs to be determined by troubleshooting each individually, and possibly in that order).
Darker than... what exactly? what it was when you first bought it? You might have a bad Inverter, or the LCD panel might need replacement. In any case, servicing it will address and fix that issue.
a disc drive that perpetually scratches and ruins discs
If both the old and the new ODD scratch your discs, there might be a foreign object in the chassis of the computer, by the ODD slot, that could be causing that. If the drive is bad, it will need to be replaced (being mostly mechanical in nature, ODDs are prone to wear-and-tear, I'd recommend a cheapie, external USB ODD).
photos - including video thumbnails - often not appearing (only a blue and white box appearing with a question mark inside)
Definitely a software issue, and though the possible causes are numerous, the most common one is permission problems with your (I'm guessing you're using iPhoto?) library.
Relatively easy to fix, too.
Not sure if by "software updates" you're referring to the OS, but a clean installation of OS X (not just of the updates and/or of the applications) can, and probably will, solve most of the issues you're describing, save for the ODD scratching disks and the screen being dark.