All the little kids are taught the Rashi that "ach" tells us that Noach was injured when the lion took a swipe at him for one time bringing his food late. Did Noach deserve to be punished just because he was late once -- did the 364 other days of the year he served the lion's food on time count for nothing? Again, lots of people talk about this, but I don't like the question. The lion cannot change its spots, or its roar and bite. Let your guard down for one day, one instant, and the 364 days go out the window pretty fast. Perhaps lion is a metaphor for the yetzer ha'ra, which needs constant minding, as in a second it can undo hours of mussar.
But why specifically do Chazal attribute Noach's injuries to the lion and not any other wild animal? The Ostrovze has a remarkable insight. Chazal (Yoma 21) tell us that during Bayis Rishon the fire on the mizbei'ach crouched like a lion; during Bayis Sheni it crouched like a dog. The AR"I explains that the smoke and fire of the mizbeiach served as a protection against aveioros. What's the difference between a watch-lion and a watchdog? If a lion is protecting you, it takes care of everything -- you can go to sleep and it will eat or scare off whatever is coming. A watchdog, however, will just bark it if sees an intruder -- you to wake up and take action. During Bayis Rishon, Hashem took care of everything; we could sit back and watch. During Bayis Sheni, the smoke and fire was there to wake us up, but it was up to us to do teshuvah and ellicit rachmei shamayim and yeshuah.
The Zohar is critical of Noach for failing to at least daven for those around him. If someone happened to see him building the teivah and took note, great -- if not, Noach didn't go out of his way to knock on the doors of his neighbors or knock on the gates of Heaven on their behalf. Noach emulated the dog; he barked a little, but he didn't bite. He lacked the quality of the lion, the courage to protect others from harm even if they themselves failed to act.
This is the lion which Chazal tell us overcame Noach.
The gemara (Meg 15) tells us that when Esther was about to take the fateful step of going to see Achashveirosh without being called for, she suddenly sensed her ru'ach hakodesh had vanished. "Perhaps it is because I called Achashveirosh a dog -- 'Hatzilah...m'yad kelev...'," Esher said to herself. She therefore changed her tefilah and called him a lion instead -- "Hoshi'aynu m'pi aryeh." What difference does it make if Esther called Achashveirosh a dog or a lion? Chazal tell us that Achashveirosh's threat to Klal Yisrael was a greater motivator to teshuvah than all the words of the Nevi'im. Esther wasn't happy about that -- Klal Yisrael should sink to such depths to need waking up by an Achashveirosh?! She therefore davened that Hashem should intervene, that the yeshu'ah should come from Him and not through the kelev-barking hisore'rus of Achashveirosh. Esther's ruach hakodesh vanished; her tefilah was too optimistic. Klal Yisrael indeed were unworthy at that moment -- they needed that outside wake up call of Achashveirosh. So Esther tried again. She asked, "If the hisorerus must come from Achashveirosh, at least Hashem save me from the lion," i.e. let Achashveirosh not be like the lion that gets all the credit for doing the job; let the yeshuvah of Klal Yisrael come at least partially through Hashem's hisoreus and intervention as well.
Why do we say Hallel on Pesach? Because the yeshuah came from Hashem alone, not through our acting or re-acting to a wake up call -- during the geulah from Egypt, "Lo yecheratz kelev l'shono." On Purim there is no hallel, as "Akatei Achashveirosh anan," we owe a measure of our redemption to Achashveirosh's wake up call as well.
(If you are not blown away by the torah of the Ostrovze after this piece, maybe you need to check your pulse.)