You'll need to remember that many (most?) online user reviews are left by idiots that know less than you do. Or at best, as much as you do. This sounds like one of those instances. These are the same people who cannot set the clock on a VCR and complain about it (blaming the VCR) -- or praise a product because it has a pretty remote (nevermind that the device works horribly).
Not having a mirror box would mean there is not an optical viewfinder. There is clearly an optical viewfinder, so that's just a bunch of crazy hogwash that you read. There's obviously a mirror in the body, based on the Nikon engineering photos shown on Amazon
Other images show an obvious pentaprism used in viewfinders.
LCDs are hard to see in most light anyway, and real photographers quite honestly should not spend much time playing in playback mode or menus. Take the photos, use your skills, and edit later. I only check an LCD for exposure quality before I get to serious shooting, or when I need to check if I have enough good shots when several assignments are back to back. (In the old days, you'd just gamble and pray you got enough as you run from one event to another in the same day!)
Looking at the LCD in sunlight (any outdoor light, to be honest) or even a well-lit indoor location is an exercise in futility, even with those special "shade" type (horse blinder style) LCD covers.
LCD-only view cameras make for a miserable experience.
A lot of consumer cameras now come with that fold-out LCD gimmick, but Nikon seems to (so far) be ignoring it in favor of more proven and sensible photo methods.
The 4x3 cameras are not as good as the standard 3x2 cameras, no. While the four-thirds system is fine for what it is, I don't see it as having any benefits to your situation. You'd be buying into a far more obscure setup.
More on the 4:3 system at http://en.wikipedia.org/wiki/Four_Thirds_system
if you're curious.
I'd stick to Nikon v Canon for your purchasing.
Sometimes I'll even suggest Sony or Pentax DSLR cameras, but this isn't one of those times.