My guess is that the Lucky Nations bonus is supposed to reflect the unique political/economic/social conditions in the main nations that rose to dominance in Europe that can't be otherwise scripted. It helps with immersion, for me at least, to be fighting the French and the British for control of Canada, or to be truly fearful of Ottoman expansion in the Balkans, and so forth. These nations were world powers, and I think it's nice that they feel like they're a tier above the rest. It's not a huge bonus, just enough to ensure that you're not always fighting Burgundy in the New World, or the Bavarian Emperor of an institution that was almost always Austrian. Plus, actually having Russia, Spain, and Great Britain form in the majority of games doesn't hurt either.
But appreciation of that depends on how strongly one feels about historical plausibility, so... to each his own.
Not entirely sure why Poland was given Lucky Nation status, though...
As for why Ironman, my only guess is that it's supposed to be for the added challenge. Defeating non-lucky nations is just too easy, and Ironman is supposed to be tough. *shrugs*
