Carry-on luggage definitely is not standardized; each airline has its own rules [1]. And if you're on a commuter plane like a Bombardier CRJ, you'll be lucky if you can fit anything into the overhead bin.
exactly - "has rules". Very few actually enforce them. And while I don't know whether the aircraft industry agreed on some standard, my pretty huge, hard-case backpack always perfectly fits in. And most airlines have actually the same rules how large items may be to qualify as carry-on: Worldwide you see the same measuring cages at check-in counters or the gate and trolleys are advertised as "carry on compatible" - both would not be possible without a maybe unwritten but pretty well established standard.
Edit: And you of course are right about very small commuter planes - but they are, IMHO not really relevant in the discussion because they are used less and less due to economic disadvantages of making something airborne for just 50 people.
True, but there's always some dumbass with a "carry-on" that they've expanded (so it no longer qualifies). Or, they don't know which direction to place the bag. Or, they jam their suitcase, plus their briefcase and jacket in the overhead, taking space that should "belong" to somebody else.
I'm far from a frequent flyer, but the number of passengers incapable of not screwing up the boarding process is amazing.
[1] http://travel-made-simple.com/carry-on-size-chart/