This would be my #1, as it would have drastically changed (for better or worse) the internet. Although, it's a psycological problem instead of a technical one. You can put a PayPal button or Stripe popup on your page using technology
today, which will usually take 10 seconds (given that the user is logged into those services, since we're assuming that this micro-payments practice is common), but it's a wild expectation that people would actually get used to willingly give up a nonzero amount of time to pay for something so small as an article.
Another problem may have been this. If everyone on the internet agreed that say, watching a video should cost $0.02/minute, then people would get accustomed to paying this amount because that's such a small cost. But it's only a $0.02 decision for a content creator to restrict or allow it to be watched for free, and if say, 5% of artists just decided to "eat" the potenential earnings, then you'd have 5% of free videos and 95% premium videos. The 5% free videos would prevent the 95% of videos from being watched, because why would anyone pay to watch a video if you can watch another one for free. Basically I'm saying that this idealization of micropayment content is destroyed by free content, whether that is a good thing or not. I do this myself with respect to Android apps, since I don't take my time seriously enough to pay the occassional $1 per app.