I have read numerous posts from various websites saying that the wizard/sorceror spell "polymorph" breaks game balance. Even Wizards of the Coast somewhat admits that. Sticking to just the core games rules, how does this spell swing game balance totally askew? Perhaps I am just not creative enough to understand the extent to which the spell can make normally challenging situations trivial. Please give examples, broken down at various ranges of character levels (from 7th level, when a wizard first gains access to polymorph, up to 20th level)?
There are some great answers here. I should have posted originally about the entire polymorph chain of spells instead of just the spell "polymorph" itself, but I think most of the answers so far took the question to mean the polymorph chain.
I do want to clarify slightly what I meant in my original question about examples, in the hopes of finding more of the "loopholes" that some have mentioned. The answer about the Avariel is a good one (gaining Fly at a longer duration at CL3 than at CL5 with the Fly spell). Any more examples of overpowered usages of the polymorph spell chain are welcome.