All of the sources I found seem to deal on some level with the investigation of human nature.
Ben Jehoaidah, Eruvin 13b explains that Israel, (he says ישראל specifically), was created as a part of the בריאה from the Throne of Glory, citing the Arizal that we were created for the glory of Gd. as opposed to angels who are a part of the יצירה. The pertinent difference between these two primordial planes is that, when coming from בריאה our non-verbal thoughts and feelings have the power to impact our surroundings. The argument was based around whether or not we should have been created within the בריאה, (notice the phrase in the Talmud uses the word נברא).
- The argument against creation within the בריאה was that since we have so very many impure or evil thoughts that negatively impact the world not having a toehold in the בריאה would make these legions of corrosive thoughts ineffectual.
- The argument for creation within the בריאה was that we would gain much, (מרויח ריוח גדול), from all of the good that our heartfelt intentions could accomplish, and any evil thoughts could be amended and purified, (I assume through repentance?), and also that they would be nullified as being in the minority, (וגם בטיל במיעותיה).
It was concluded that we'd be better without a stake in the בריאה, like angels, because our negative feelings and thoughts are overwhelmingly many. In this source, we see the discussion and evaluation of human nature as a powerful thing in and of itself, even without action.
Etz Joseph, (in Ein Yaakov, Eruvin 13b), quotes the MHRS"A on Makkos 23a, who explains that the Torah is broken down in to positive 'do' commandments and negative 'don't' commandments. The argument was grounded in this frame work.
- The argument against creation was that while on the the one hand we lose the opportunity to perform positive commandments, we at least are guaranteed the fulfillment of the negative ones by our non-creation and by extension our inaction. However if we are created, we run the risk of transgressing the negative commandments, which would counteract any reward we might gain from positive commandments, (יצא שכרו בהפסדו).
- The argument for creation was that by not being created, we are guaranteeing that we will not fulfill any positive commandments. Thus all of the reward we get from fulfilling the negative commandments would be lost while filling the (hypothetical?) lack of reward from the neglected positive commandments, (יצא הפסדו בשכרו). However if we are created, we gain the opportunity to reap reward from all of the positive commandments.
We see again human nature, this time in the context of risk and reward. In the end, it came down to numbers. The commandments were formally enumerated, (perhaps for the first time which is why it took so long?). There were found to be 365 negative commandments versus 248 positive commandments. Thus it was concluded that what we stand to gain comes in less number and variety than what we stand to lose, and thus we would be better off not running the risk of creation.
The Anaf Joseph, (Ein Yaakov, ibid), brings two possible understandings.
First, he explains that we could have been created by way of an 'intermediary', like the rest of the animals, who were brought forth by the globe, (גלגל, By way of either earth or water? He seems to only mention earth, perhaps due to a focus on humans?). In other words, Gd could have commanded that the earth bring us forth just as he commanded it to bring forth plants and animals. Or he could, (and did), create us directly. The argument was about manner of creation.
- The argument against our direct creation by Gd was that being created on a lesser level, by way of an intermediary, would make our sins and punishments less severe.
- The argument for our direct creation by Gd was that being created directly by Him would cause him to be more lenient and merciful with us , so to speak, as opposed to indirect creation which would result in immediate destruction.
So the argument here was based around human nature when it came to sin, and the examination of the consequences of sin. The agreed upon conclusion was that It would be better for us "to not be created", (I assume directly by Gd based on what comes next), because being created on this level, work of the hands of the Creator of all, results in much more severe ramifications of our sins, (I speculate that the alternative argument was dismissed because it was also concluded that sin does not result in immediate destruction, and that even the extra mercy, so to speak, that we enjoy from Him is strained, so to speak. See this 2000+ year exile for reference).
His other possible understanding does not detail the arguments, just the point of contention and the conclusion with its reasoning. He says that בריאה specifically was discussed. בריאה is our creation as is, which requires us to to elevate ourselves to a spiritually whole plane of עשייה, with free choice. This would be opposed to only experiencing עשייה, that is existing in a way that it is our nature to fulfill Gd's will without inner conflict. It was concluded that just עשייה would have been better, because the paragons, the בני עלייה, who actually attain this ascension from בריאה to עשייה are few and far between.
I am speculating, but it seems that the Sages struggled with the path of ascension's merit and increased reward versus its extreme difficulty. On the one hand, arriving at עשייה from בריאה would find us more richly adorned than if we began at עשייה. On the other hand, the struggle of ascension could prove to be too much for us, leaving us right where we started or lower, so why not just begin from עשייה, with guaranteed moderate sustenance and reward? After a statistical analysis of human nature, the Sages concluded that for humanity as a whole, עשייה would have been a better starting place.