Exhibit A: Just about every Polymorph-related thread for previous editions of D&D.
I don't know if that was in support of or against a paracritter option for (critter) form, but since you bring up D&D...
If I recall correctly, pretty much every version of shapechange that allowed supernatural forms came with the caveat that you only gained their natural, non-magical physical abilities. So, you could take the form of a dragon, but you would not get its breath weapon, but you would get its claw/claw/bite/tail/wing attack. I'm less clear on if you got flight, but I don't think so (I hated playing magic users of any stripe because the spell memorization/slot system rubbed me the wrong way).So for SR, I would think Dragon form would allow you to look like a dragon and gain their bite and claw attacks, but not gain their elemental attack, dragonspeech, hardened armor, or anything else that isn't normally a feature of a mundane critter (effectively armor, natural weapons, Enhanced Senses, and winged flight). Becoming a Shark or other sub-aquatic animal would grant you the ability to breathe water (or hold your breath a really long time in the case of dolphins and whales) as well as an enhanced swim speed, though the stat block doesn't indicate that, nor does it indicate the ability to fly for critters that should have that ability, so these things aren't so much powers as innate qualities I guess.
Of course, with the +/-2 BOD requirement of the spell, you aren't going to see much in the way of Dragons (though a 13+ BOD Troll could pull off a Feathered Serpent form) so treat the Dragon example as more of an extreme case. I'd also rule that it would have to have a physical form, so no ghost/fog/ball of light forms, etc.
As likely evidenced by the forum posts you mention, there may be edge cases (this paracritter has flight, but no wings, do I still get flight? No as it's a supernatural effect, etc) but it is not broken or otherwise impossible to implement sanely.
