"Does requiring people to sort their trash actually work? What sort of feedback and enforcement is in place?"

No it doesn't. I live in so-called 'progressive' Holland where separate waste collection has been done for years. It's not efficient now any more (in 2013 we have better waste separation methods as was had in the 1990's) but people have become conditioned to 'separating waste = green' that it's hard to reverse now. Plus, the whole infrastructure exists now anyway, so the switching costs would take another decade to make back - an investment for which there is simply no room in these times.

That sounds like it is working, but you don't like it.

Oh sorry I was making another point than what was being asked. Yes it works, but what I am arguing is that it isn't the most efficient way. I do like it in the sense that it was the only thing we had for the last 20 years, and I obviously support separating waste (be it at the consumer level or centralized) for environmental purposes, but it's hard to tell what (in hindsight) was best: that we had separation at consumer for the last 20 years, but that we will have to live with a more inefficient system for the next decade(s); or that we would have postponed separate collection 20 years ago until let's say 10 or 5 years ago, and started with the more efficient (centralized) separation system.

It's an interesting question, and very difficult to answer. Like climate studies, you'd need to calculate so many side effects, that your answer might be different depending on the context you take into account. I've discussed this topic quite extensively with colleagues in urban planning and environmental sciences in countries across the world, but nobody knows (obviously) and people's preferences tend to align with what they have or have worked on. Personally I tend towards the automated / technology solutions (i.e., centralized), but I can't tell if that's because of a cognitive bias or from an increased ability in estimating these sort of things from experience, so that basically nullifies my opinion, too.

It's water under the bridge anyway.

Yes, to the point that people take pride in it. Colleagues suffer minor, humorous ridicule for putting something in the wrong bin and there are conversations about plastic type 5 vs plastic type 2. One guy is designated "Recycle Man" for leading office efforts. Under most desks in the office are two bins, one for cardboard/paper/etc. I don't know anyone who even thinks about it as a chore now.