That is a tough one. I have been in the industry for 10+ years and a lot of it in the higher end custom home side. To me a couple of things that standout as super cheap are jump ducts (basically they didnt bother to put a return air duct in a room and just put a whole in a wall with grills on either side to allow air transfer), hollow core interior doors (you can hear/feel the difference if you knock on a hollow core door versus a solid door), all stucco exterior (cheapest clading material where I live), sometimes upper floor water pressure is almost non-existent and they dont put in a booster pump, and on and on... I feel like it really varies from project to project. I have seen some where they actually spent a decent amount on the cabinet drawer/door fronts, but then didnt bother to spend a couple of hundred more on good undermount drawer guides. Others where they didnt bother to think about layout/flow in a kitchen (aka the island looks great, but if I open my fridge I cant squeeze by the island and the door to actually get anything out of it). I think generally a lot of what people are trying to build today qualifies as "modern" but to me it just looks/feels cheap. To truly do modern well is very expensive. I dont particularly like the look, but thats just my opinion.
Older construction is another area entirely. When flipping houses became popular a lot of people got into the industry who didnt know what they were doing, or they went about trying to do it as quick and cheaply as possible. I have heard that the less you know about a place you are trying to flip the better. AKA if I dont open up any walls to see if the electrical doesnt actually meet code, then I dont have to spend a bunch of money replacing it! Or I didnt get a structural engineer to look at the foundation so I dont know if there are any issues with it and therefore I dont need to disclose anything to a potential buyer.
I generally have a different opinion. I am somewhat in the industry (construction side) and I feel that generally most realtors dont really have much knowledge of what they are selling. Furthermore, it is relatively easy to get a real estate license (at least where I live) and the handful of acquaintances that have gotten their license really dont know more than the average person.
I actually find the lack of knowledge of most realtors very surprising (not that there arent some really good ones out there). When we were working with one to find a home, we would walk into a place and our realtor would look through the listing sheet and be like "oooh they supposedly have a good quality insulation in their walls", I would then need to explain to him what an R value means and that no they truly didnt have good insulation and what they were using as a marketing point was in fact the minimum required by code. I would then point out a few other items (jump ducts, hollow core doors, etc) that would make me feel like I was walking into a Habitat for Humanity type of home, not a $600,000+ residence.
Buying a home is the largest single investment people are likely to make in their lifetime and they have almost 0 knowledge about what they are buying and do almost no research into it other than the location. They then rely on input from someone regarding what they are buying that has very little additional information other than comparable sales and "knowledge" of the market. I wrote a business plan a little while around this, but never took past just putting some ideas on paper.
Older construction is another area entirely. When flipping houses became popular a lot of people got into the industry who didnt know what they were doing, or they went about trying to do it as quick and cheaply as possible. I have heard that the less you know about a place you are trying to flip the better. AKA if I dont open up any walls to see if the electrical doesnt actually meet code, then I dont have to spend a bunch of money replacing it! Or I didnt get a structural engineer to look at the foundation so I dont know if there are any issues with it and therefore I dont need to disclose anything to a potential buyer.