What is the common understanding of all enlightened people?

They are unbound: they do mot cling to pleasure or push away pain.

They have perfect virtue: the five precepts if not the eight.

An anagami and arahant have perfect samadhi

They all have seen the four noble truths in their own effort.

One answer might be this from verses 183 through 185 of the Dhammapada:

183. Not to do evil, to cultivate merit, to purify one's mind - this is the Teaching of the Buddhas.
184. The best moral practice is patience and forbearance; "Nibbana is Supreme", said the Buddhas. A bhikkhu does not harm others; one who harms others is not a bhikkhu.
185. Not to revile, not to do any harm, to practise restraint according to the Fundamental Instructions for the bhikkhus, to be moderate in taking food, to dwell in a secluded place, to devote oneself to higher concentration - this is the Teaching of the Buddhas.

Or you might see What teachings do all schools of Buddhism share?

Or maybe there's the three or the four dharma seals.

I think these would all be good answers but perhaps not not the ultimate answer -- because "what about compassion?" for example.

If you were to ask instead, "what is the common skill of all skilful people", then you might see that the question is senseless or difficult to answer or conditional -- if "what is skilful" depends on or varies with the circumstance, then maybe "what it's good or enlightened to understand" also varies.

Incidentally perhaps Buddhism doesn't talk much about being "enlightened", exactly -- instead the word used in the Pali suttas might be something more like "liberated".

The common understanding of enlightened people is that all the trials and tribulations we experience in the world are products of the unbridled operation of the thinking mind, not features of the world itself. The thinking mind turns situations into problems, occurrences into worries, behaviors into offenses, and spirals through cycles of emotions and thoughts because what is doesn't agree with what it thinks should be. To paraphrase Dylan Thomas ("A Child's Christmas in Wales"), a lot of the activity of the human mind is like blowing 'a whistle to make the dogs bark to wake up the old man next door to make him beat on the wall with his stick to shake our picture off the wall,' and then complaining about the picture falling off the wall and worrying about how to fix it. If the mind is calmed so that we lose that mind-urge to blow the whistle in the first place, the world is a calm, peaceful, beautiful place; If the mind is agitated, those whistles get blown, and all the subsequent chaos ensues.

The four noble truths is the absolute common ground.

Their meaning are not just subjects of discourse or semantics, but ultimately for investigation, or "discernment" and "pondering":

But then there is the case where some clansmen study the Dhamma... Having studied the Dhamma, they ascertain the meaning of those Dhammas with their discernment. Having ascertained the meaning of those Dhammas with their discernment, they come to an agreement through pondering. They don't study the Dhamma either for attacking others or for defending themselves in debate. They reach the goal for which people study the Dhamma. Their right grasp of those Dhammas will lead to their long-term welfare & happiness. Why is that? Because of the right-graspedness of the Dhammas.

https://www.accesstoinsight.org/tipitaka/mn/mn.022.than.html

As others have already answered, the four noble truths are the wisdom held in common by the teachings of the Buddha that lead to enlightenment. As the Buddha said in his first discourse,

"As long as my knowing and seeing how things are, was not quite purified in these twelve aspects, in these three phases of each of the four noble truths, I did not claim in the world with its gods, its Maras and high divinities, in this generation with its monks and brahmans, with its princes and men to have discovered the full awakening that is supreme.

(Nyanamoli, trans.)

Sariputta's attainment of sotapanna is a good example of how the four noble truths are both necessary and sufficient for enlightenment, as what you call cause and effect is actually a concise teaching on the truths, as the commentary relates:

ye dhammā hetuppabhavāti hetuppabhavā nāma pañcakkhandhā; tenassa dukkhasaccaṃ dasseti.

"Those dhammas that arise based on a cause": Those things that arise based on a cause are the five aggregates. By that he showed the truth of suffering to him.

tesaṃ hetuṃ tathāgato āhāti tesaṃ hetu nāma samudayasaccaṃ; tañca tathāgato āhāti dasseti.

"The cause of them was told by the Tathagata": the cause of them is the truth of the origin of suffering. "And the Tathagata spoke that," he caused him to see.

tesañca yo nirodhoti tesaṃ ubhinnampi saccānaṃ yo appavattinirodho; tañca tathāgato āhāti attho. tenassa nirodhasaccaṃ dasseti.

"And the cessation of them": the cessation by non-manifestation of both those two truths - "And the Tathagata spoke that," is the meaning. By that he showed him the truth of cessation.

maggasaccaṃ panettha sarūpato adassitampi nayato dassitaṃ hoti, nirodhe hi vutte tassa sampāpako maggo vuttova hoti. atha vā tesañca yo nirodhoti ettha tesaṃ yo nirodho ca nirodhupāyo cāti evaṃ dvepi saccāni dassitāni hontīti.

But here the truth of the path is not explicitly shown; it is however shown implicitly. For when cessation was said, the path leading to it was also said. Either that or: "And the cessation of those" here means "the cessation of those and the means to cessation," thus the two truths are both shown.

Good evidence of the need to teach the four noble truths to attain enlightenment exists in the Dhānañjāni Sutta (MN 97), where Sariputta fails to teach the four noble truths and the listener goes to be reborn in the Brahma world. As the commentary says:

kālaṅkato ca sāriputtāti idaṃ bhagavā “tatrassa gantvā desehī”ti adhippāyena theramāha. theropi taṃkhaṇaṃyeva gantvā mahābrahmuno dhammaṃ desesi, tato paṭṭhāya cātuppadikaṃ gāthaṃ kathentopi catusaccavimuttaṃ nāma na kathesīti.

"And he has made an end to time [i.e. died], Sariputta": this the Blessed One said to the elder with the intention, "having gone there, teach him." And the elder, at that very moment having gone, taught the dhamma to the great brahma. From that moment on, even when teaching a four line verse, he never again taught anything devoid of the four truths.