When one starts sharing decks it frequently happens that people violate those rules and try to learn Anki cards for material that they haven't learned beforehand.
Fortunately there are enough exceptions to Wozniak's guideline that violating rule 2 can often be beneficial. For me this applies to any information that I am able to understand (rule 1) from the terse information in the cards themselves. In the same way I tended to learn most efficiently from practice exams when studying.
For material that is too complicated (or simply insufficiently specified) to learn from the Anki deck I do honor rules 1 and 2. This means I keep the deck in my active reviews only if (and when) I am sufficiently curious about the subject matter that I will naturally be inspired to look up every term I encounter and do not understand. Just In Time learning is viable.
Spaced repetition is optimal for recalling factual information. It won't necessarily teach you anything that you haven't already learned. It helps you retain knowledge, and won't necessarily help you develop skills. But, within the domain of factual information that you can already comprehend, spaced repetition systems are pretty optimal. So, if you want to train your brain on a bunch of Spanish-to-English sentence translations, or stock market tickers, or definitions, or sample questions, you should use something like Anki.
Once you start using spaced repetition, you learn that one of the biggest limits is the card-making process. Making your own cards is time-consuming, although experience will make you much faster. Experience will also teach you what makes a better card. The 20 Rules of Formatting Knowledge pretty much spells it out for you, but I still had to make my own cards, find the sticking points, and edit them until I got a good sense for proper context and suitably short, distinct answers.
You can find other people's shared decks and skip the card making process yourself, but not without some new problems. First, you are going to learn more making the cards yourself than studying someone else's. If it's subtle material, you can make cards that fill in the gaps in your particular understanding. But if someone else studies something and makes cards to fill in their own gaps, that means that what you're studying may not cover material that you don't know you don't know. It would be nice if everyone's shared decks were a completely thorough treatment of the material, but, alas, it is not so. And the only way that you can tell is by comparing the deck and the material during your own studying.
Shared decks also just aren't all that good sometimes. Someone, I can't recall who, wrote a script to scrape the entire LW wiki and then cloze-delete the title from the article. I appreciate the idea of SRSing the LW wiki, and scripting the whole thing was undoubtedly really efficient. However, the result was usually question and answer text hundreds of words long, with tables of content in the middle, and probably too many cards of insufficient value.
Despite their problems, I think that shared decks have way more potential than their current use suggests. A well-crafted deck that gives its subject matter a thorough treatment could be more valuable than a textbook, and about as difficult to compose. But, looking at some of the best Anki decks I've come across, it will likely take more than one person to get such a deck off the ground.
Anki's .apkg files are sorta unwieldy to edit collaboratively, because there's not really a way to merge edits from multiple contributors. Luckily, we can export and import decks as text, and use version control like GitHub to do the same thing. With a GitHub-hosted collaborative deck, a team of people studying a textbook, like Thinking and Deciding, could all make flashcards as they go, add them all to the same deck, remove redundant cards, standardize the layout, tag cards appropriately, and share them with whomever else comes along. Then, anyone else who wants to study the textbook has a high-quality Anki deck to use in conjunction, and if they know how a question can be asked better, or if they find an error, or if the seventh chapter didn't really get much coverage, they can contribute to the deck, too.
This huge list of material put together by Louie Helm should be Anki-fied. Hopefully we can unite the efforts of many autodidacts and start to curate decks for each of the areas covered. Maybe a group of friends is about to work through a course on Quantum Computing or Set Theory. The rest of LW would benefit from their work making flashcards, but especially so if they leave the project open to collaboration.
So, the things needed to move forward: