What technology has mass adoption ruined?

Pesticides. If they had been used to control the environment of small areas it would have been largely fine.

Using pesticides to control the environment of large swathes of land was bound to cause all sorts of problems, and it did, well beyond the areas they are used on.

Cars. When everyone has one then traffic and sprawl cancel the speed advantages. Without all the highways and parking lots and overbuilt roads you could walk in the same time.

There are so many. Some might even wish there was some of kind of IQ-test and permanent branding in social media. In the old-time Usenet everybody was associated with an academic institution, which already implies some mental capabilities perse.

Could you explain what you mean by that?

Would leaded gas and CFC refrigerants be included? Or whale oil, which used to be a major energy source?

Or the overuse of antibiotics, leading to antibiotic-resistant bacteria far earlier than might otherwise have been?

Personal computers.

The Web.

General-purpose computing