[MENTION=46713]I use a variant of theatre of the mind, with abstract positions, so yes, fiction position can be affected, but position by rules is untouched. Also, there's a standard assumption that the base character has all the properties of an average person, so with that in mind...
By 'fictional positioning', I merely mean 'What is stated to be true about the fictional universe'. So, the spell changes the fictional positioning in that, "You now look like a grapefruit/canary/barrel of beer/donkey/tavern wench.", is a statement of truth within the shared imaginative space.
1) NPCs would react differently, but I'm not sure how that doesn't qualify as fluff.
Generally speaking, anything that could significantly and regularly effect the fictional positioning is not fluff.
So fluff would be like:
Player: "My M-U has a thing for the color green. Can his burning hands produce green fire?"
DM: "Sure. As long as it's just fluff, you can make your spell effects look however you think suits the character. Just understand that fluff doesn't ever change anything."
Crunch would be like:
Player: "My M-U is supposed to be a master of shadows. Can his burning hands produce no light?"
DM: "Errr.... that sounds like you are trying to leverage what should be fluff into a real mechanical advantage. There would be a real and consistent advantage to having a spell that was extra stealthy. Sorry, but I won't allow that without spending some sort of tangible chargen resource."
But notice the difference in the second case is really 'only how NPCs react', that is to say, whether they do react. So at the very least, being able to assume the appearance of something else, even if it only effects how NPCs react, is not just fluff. If you are being chased by a monster through the woods, and you assume the form of a pine tree, if you have really assumed that form then you are now suddenly much harder to detect and the monster may well react by wandering off. A spell that causes a monster below genius level intelligence to wander off is not a spell without mechanical benefit!
2) Under the average person rule, you could fly anywhere an average person could walk (or run). Same with swimming, and no water breathing.
So this spell gives you the appearance of being able to fly or swim, but not actually the ability. That is (for the most part) actually just fluff. Any non-fluff effect of appearing to do some form of movement you are not could be covered under the above problem that the ability to assume any guise is not a trivial ability.
However, as soon as you do not get the abilities of the form you assume, you are no longer shapechanging. You are either polymorphing or more likely, using some sort of illusion magic. So you do not have a 1st level shapechange. You might not have even a 1st level polymorph spell.
You're probably getting the point by now, but...you couldn't really turn into a cockroach, since it's significantly smaller than an average person.
No, but now I do get the point. This isn't polymorph or shapechange. This is a slightly OP disguise self. It's still OP, but it's not nearly as OP as a 1st level polymorph or shapechange. Polymorph by definition means that you can actually assume the form of something, and shapechange generally means that except otherwise stated you have all the abilities of the form you assume. Given that your polymorph lets you assume the form of something, but only the form of something that has very nearly your same form, it's not what people normally think of as polymorph.
Compare with the 1e 'Polymorph Self' spell, which gave you the form only of the thing you assumed, but not its abilities or attack modes. That was a 4th (or 5th, can't remember at the moment?) level spell (and well balanced) just on the basis of letting you fly like a bird and look like a bird.
And if that person could squeeze out of manacles, the new polymorphed shape could do it too.
Which again suggests that what you have here is not polymorph at all, but rather a some what less limited version of 'disguise self'. If I change into a long and slender snake, not only am I not gaining the actual abilities of the form such as same a venomous bite (as a shapechange spell would allow), I'm not actually gaining the form of the snake either! My new form will be just as limited as my old form and in exactly the same ways. So, this isn't even a polymorph spell because while my appearance changes, I don't actually get the benefit of 'multiple shapes'.
This is not a spell that is 'just fluff' then. And if there is no mechanical limitation to the effectiveness of the disguise, then not only is this not just fluff, it's EXTREMELY potent mechanically. Consider the 3.0e rules for disguise self, that gave you a +10 enhancement bonus to disguise checks. Or imagine something similar in 5e that said, "You gain advantage on disguise checks." It's not less mechanical to assert, "You can assume a disguise." Just because you don't reference the rules regarding disguises doesn't make your spell less crunchy. What it means, to the contrary, is that your new spell effectively says, "You gain a +infinity enhancement bonus to disguise checks." That spell is inherently more crunchy goodness than one that does provide a limit!
5) Turning into anything edible-looking would still get you snatched by a dragon. A small bale of hay, on the other hand, might be safe. And flammable.
Wait... can I change into forms that aren't flammable then? Because that would be a huge mechanical advantage.
