Hanuman a passive spectator in the tussle between Vaali and Sugriva

Hanuman with his immense strength could have destroyed Vaali easily. But he was a mute spectator to the tussle between the brothers. 

This might be because Hanuman was not a king and couldn't therefore challenge Vaali who was a king.

Later, in Uttara Kandam, Rama discusses this with the sages.

Shri Rama: When Vali and Sugriv became enemy of each other; then as forest fire destroys trees, why did Hanuman not destroy Vali in the same way? This is difficult to understand. I believe that Hanuman did not remember his strength at that time. That is why, he was left just to see his beloved Sugriv suffering in such a situation.

Shri Agastya: O' Shri Rama! Whatever you say about Hanuman is all true. There is no match to Hanuman in strength, intelligence and speed. O' destroyer of enemy, Shri Rama! Sages, whose curses are never vague, had cursed Hanuman that despite having the strength; he would be forgetful of the same.

Sugriva claims Tara! (Deceased Vaali's wife)

After Vaali's death, Sugriva claims Tara! 

This is very surprising as it happens just a few months after Vaali's death. Tara appears to have been under tremendous grief on hearing Vaali's demise and wanted to leave the world. 

Sugriva wasn't punished as this wasn't considered heinous according to Vanara dharma - where a vanar can marry deceased brother's wife with her consent.

(Vaali claimed Ruma when Sugriva was alive. This was considered sinful)

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Exquisitely featured Tara went to the proximity of Lakshmana with a waddling gait, tipsily flustering eyes, rrivière-strings of golden cincture dangling, and with a sticklike gracile body humbly bent. [4-33-38]

On observing the wife of the king of monkeys, the great-souled son of the king of humans stood aside unemotionally, and because of the presence of females he backed down on his anger with his face cast down. [4-33-39]

"By the beneficence of Rama, oh, the enemy-scorcher Lakshmana, now Sugreeva is privileged with reputation, everlasting kingship of monkeys, more so with Ruma and me as well. [4-35-5]

Vaali violates Ruma (Sugriva's wife)

After coming back from vanquishing Maayavi, Vaali is enraged to find Sugriva decorating the throne and accuses him of conspiracy. He then throws out Sugriva out of Kishkinta and snatches his wife Ruma and violates her. 

Rama punishes Vaali for this despicable act of transgression to a lady whom he should have considered as his daughter-in-law

Vaali's boon

Vaali's boon that he will take 50% of power from his opponent who ever challenges him.

This fact is not mentioned in Valmiki Ramayana. 

This appears in Kamba Ramayana, where Vaali discloses this to his wife Tara in a conversation

Rama advances near Jatayu to kill him mistaking him for a Asura.

While searching for Sita, they find a fallen bird drenched in blood. Rama speculates this giant eagle might have tried to swallow Sita and hence struggling in the process. He advances near the bird with the intent of killing it. 

Jatayu in it's pitiable condition implores that it's not fair for Rama to kill him as he's already about to die after been vanquished by Ravana. 

Jatayu then details about the big fight with Ravana and Sita's kidnap.

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Then Rama beheld Jataayu who is akin to a mountaintop in his gleam, and a best beatific brave bird among all the birds, but bloodstained and buckling on earth. [3-67-9b, 10a]

On beholding him who is like a mountaintop in his gleam, Rama said to Lakshmana, "This one has gluttonised Seetha, the princess from Vaidehi, there is no doubt about it... [3-67-10b, 11a]

"It is apparent that this demon skulks in the forest in the semblance of an eagle, and having gluttonised that wide-eyed Seetha he is now reposing comfortably. I will eliminate him with deadly arrows, whose arrowheads blaze away and which shoot off straightly..." Thus Rama declared. [3-67-11b, 12]

On saying thus and on readying a crescent-shaped razor-sharp deadly arrow on his bow, that ireful Rama rushed towards the eagle, as though to a-tremble the whole of the earth that which is within the perimeter of oceans. [3-67-13]

But that bird spewing up frothy blood pathetically appealed to such an ireful son of Dasharatha, namely Rama, in a pathetic tone. [3-67-14]

"Oh, boy Rama, blest is your eternality, for whom you are searching as with the search of a rejuvenescent herb in the vast of forest, Ravana stole that lady and my lives, too... [3-67-15]

Vaali and Sugriva are half-siblings

Aruni served as charioteer of Surya, the sun-god. Aruni's female form and Surya gave birth to Sugriva. While Aruni's female form and Indra gave birth to Vali. Both the children were left in the care of Gautama and Ahalya. 

(There are other legends that say that they were born to Ahalyai through Indra and Sun (in a story that was mentioned earlier) and Gautama cursed them to turn as monkeys when he learnt they were not his progeny.)

In any case, the fact that they are half-sibling doesn't change. They were given by Indra to Riksha - the childless monkey king of Kishkinta.

Lakshmana Rekha

After the bitter conversation, Lakshmana tries to console Sita, bows to her and leaves in search of Rama. There is no mention of Lakshmana drawing any line or Seetha crossing that line (in any of Valmiki/Kambar/Tulsidas Ramayana)!

This incident is mentioned in Ananda Ramayana.