there’s a thing called a “bed wetting alarm” that helps with night-training (little kids learning to not pee in their sleep). As far as I can tell, everyone who actually uses one seems to think it works really well and was really happy they did so. It’s simple and inexpensive. But nobody uses it as Plan A for night-training, or even Plan B, or even Plan C. People seem to only use these things as a last resort when they’re absolutely desperate. Like, I suggested it to my wife and she said something like: “oh, it’s totally fine that night-training takes a while, our kid is still young, don’t worry, you shouldn’t feel like you’re desperate”. And then I brought it up to my mother, and she said the same thing. And then I brought it up to the pediatrician, and she said the same thing. None of them offered any actual reason not to buy the damn alarm. Worse than that, it was as if it didn’t even occur to them that offering such a reason might be warranted. The whole experience was surreal.

A big one that comes to mind is embryo selection for intelligence.

Brain implants and/or genetic modification via viral vector for improvement of brain function (e.g. treating intractable depression). 

Infrared dispersing paint/materials for roofs in hot climates.

CLARITY for room-temperature-stable brain tissue preservation that allows for repeatable non-destructive imaging.

The 'Silver Lining' ocean water cloud spraying for reflecting sunlight over tropical oceans in order to reduce global warming.

Biochar. Burning cellulose-rich agricultural wastes into charcoal, generating energy and sequestering carbon (the inorganic charcoal lasts tens of thousands of years in soil without breaking down, unlike just composting the cellulose which causes it to lose carbon to CO2). The resulting charcoal can then be impregnated with ammonia for a slow-release fertilizer which delivers fertilizer more efficiently to crops and thus reduces the total amount of fertilizer needed.

Treating burying used plastics in deep well-designed landfills as a good thing for the environment, because it's an efficient way to do carbon sequestration.

Having a country that not only doesn't limit, but actively recruits and sponsors immigration (including granting full citizenship rights without requiring previous citizenship be revoked) from anyone who scores above threshold on an IQ test or has a track record of impressive academic or entrepreneurial achievement and is at least a decade below retirement age. They are valuable users to be acquired! Countries should vie for these like apps vie for users! 

Land Value Tax

Approval Voting (or, failing that, then at least Ranked Choice voting)

Better dam infrastructure management, like letting high-turbidity flows pass through during heavy precipitation events to reduce sediment deposition behind the dam.

and many more not coming to mind right now. This is kind of a pet peeve of mine. So many good ideas out there that I have no idea how to get the world to start using.

Checklists. It seems obvious that they work, but I almost never get a literal piece of paper with boxes to check, even where doing so would be simple and cheap, especially compared to the potential costs of skipping some step and doing it wrong.

What is distinctive to me about Zoom is that the technology was:

• Mature
• Cost no money
• Was widely available and fairly intuitive for the average person to use
• Had no regulatory barriers or moral issues
• Can be used in full form by mutual agreement among one or a few people (technically, you can approval vote on anything with your friends, but the central use case is for elections and so it would not count for this criteria)
• Saved a lot of money and time
• Had an immediate payoff
• Played relatively well with the existing format of meetings
• Appealing largely to people in the developed world with high access to information about useful new products and services
• Potential daily users in the hundreds of millions

I think your analysis is correct that it really was just a cultural normalization step that was preventing us from adopting it at mass scale.

I also think that technologies with all these properties are very rare. In fact, I am about 75% confident that no other equivalently long-neglected technology or product exists that also has all these properties. In other words, my one-word answer is "None."

I am a poor recent grad, so I can't stake a lot of money on this. But just to make it fun, I'll donate $10 to the Against Malaria Foundation if anybody can propose a technology that is "like Zoom pre-COVID" in terms of being neglected while having all the attractive properties I list above (by my lights, but I'll be somewhat generous in interpreting proposals - digging up technologies that are fairly compatible with this framework is more important to me than hanging on to my $10).

I'm not counting checklists or landfills because they are already mentioned here and because I think the benefit to the average person probably still isn't that great in most cases.

Here are some examples that aren't free to adopt, but don't cost much extra compared to business-as-usual when it comes time to build or buy something similar, and/or pay for themselves over time.

Waste gasification to make hydrogen, or syngas, or fuels, or chemicals. Renders hazardous and/or non-recyclable material into a valuable commodity at net profit, while freeing up landfill space. Starting to pick up steam slowly, finally. (And before you toss this in the "not available to individuals" bin, there are companies making and selling models that fit in a pickup truck and are scaled to provide a few kW of electric power to a household, farm, or small business. If I had one I'd never have an electric bill and would produce almost 90% less trash by volume).

Having air-source heat pumps for more efficient heating and cooling, or combining heating/cooling infrastructure to save space. People still think this is only viable in a limited set of climates, but that's much less true than it used to be. Becoming more valuable as more people get home solar panels and otherwise cleaner sources of electric power, and as movement away from oil and gas for heat continues.

Drawing/annotation/virtual whiteboard in video calls. If my work laptop had a touch screen and stylus I'd use this all the time.

Prefab construction, especially for houses. Labor is expensive, centralized production is more efficient in labor and materials, quality monitoring and continuous improvement are much more reliable in a factory, and it's mostly zoning/permitting/inspection rules + popular perception of the low quality of existing "mobile homes" that hold this back. See things like log  home kits, some of them are really well designed, full size houses with all different floor plans and high quality materials.

Composting toilets, especially in dry climates.

Routine practice of certain schemas, such as rerouting from negative motivation to the isomorphic positive representation of the same contents.

There are a few solutions that have increased capital costs while providing both more convenience and reduced CO2 emissions. 

1. Orbital Systems Shower - Using sensors that measure how dirty water happens to be + Nanotech filters to reuse 90% of the showering water. In those areas where saving water is important, that framework is much better than the status quo.
2. Energy recovery ventilation in residential buildings. There are installation costs, but the air quality including CO2 is better. Fewer fossil fuels need to be burned for heating.

Less capital costs:

1. Automatically open/closing blackout curtains.
2. InstantPot

Medicine (in desperate cases it makes sense for consumers, but it's more a matter of research):

1. Given that treatments based on peptide that exist in the normal human blood stream can't be patented, there seems to a bunch of substances that work but aren't used in medical practice.
2. Phage therapy. Given that you want to use a lot of different phages is different situations approving a single phage doesn't bring you far and as a result it's hard to make the business model work in the current regulatory environment. 

Glasses-free 3D displays.