Good questions! I'll start. The ACA is an effort to expand health insurance coverage and has resulted in a significant reduction in the number of Americans uninsured. According to Wikipedia, health care spending increases have slowed since its implementation and the CBO also reports that it will reduce the budget deficit, while repeal would increase it.
One of the most popular ways it expands coverage is by requiring insurers to cover people with pre-existing conditions. This is provided for by the individual mandate, which 'requires' (with a soft tax punishment) people to buy insurance. These provisions work in concert. Thus, everyone cannot wait until they are sick to get on guaranteed insurance, which would collapse the market. That would be like waiting until you crash your car to buy auto insurance and then forcing insurers to cover that. There are two strong criticisms of the mandate, from opposite camps: one, that it doesn't go far enough to compel people to buy insurance, and two, that it exists at all. But overall, the mandate, working in concert with the pre-existing conditions requirement, form one of the central planks of the ACA effort to make sure everyone gets covered.
One of its most effective elements has been Medicaid expansion to simply put more lower-income people on health insurance. However, states are not required to implement this (which would have been funded by the federal government, albeit I believe temporarily) and as of last fall 19 states still had not. What this means in those states is that low-income families caught in the gap (sans Medicaid expansion) are pushed onto the healthcare exchanges instead. Like the above, there are at least two camps: those who feel expansion didn't go far enough or that it shouldn't have been optional and those who don't want the federal government involved at all.
A serious remaining failure of the ACA is the amount of people that are still left uninsured. From the left, this has led to vocal pushes to either tweak the ACA (increasing the mandate penalty, Medicaid expansion, etc) or to forge ahead with single payer / Medicaid for all. From the right, this has given them the rhetorical ammo to push for total repeal. [Disclaimer: I now heavily fall in the 'single payer' camp although I am happy to fight for just preserving the ACA, given the alternative.]
I'm going to mention premium prices and the "one-time" premium spike of 2016 as well as the precariousness of the current exchanges to political uncertainty (e.g, an administration can certainly try to sabotage the ACA through neglect if not active efforts, and this one appears to be doing at least one of those) without delving into them. It's another big topic and I'm sure others (such as JJ/ED and BRB) will chime in. This is a worthy area for further reading, especially for me.
Overall, it was sweeping and significant legislation. The central philosophy underlying it is that healthcare is a human right. The ideas used to implement it borrow heavily from market-conscious conservative approaches. In this way it kind of leaves everyone ticked, but it was a major makeover from the status quo and is now proving awfully hard to toss aside.
Apologies in advance for the incompleteness / possible lack of rigor here. Hopefully this is all big picture enough to not require more detailed sourcing yet. I'm by far not the most well-read on this, either (go back to a few years ago in any ACA discussion here! I think I prefaced every post with such a qualifier) so I'd definitely welcome any corrections to anything I've said. A large part of my reading/understanding of this topic has developed from 2015 onwards, when it became clear that it was going to be a key consequence of the 2016 election. To be fair, this was also true in 2012 -- I just paid less attention in general back then.
