The MongoDB user group (https://groups.google.com/forum/#!forum/mongodb-user) has a great archive of user questions about features and long-standing bugs. It is a place to go when something doesn't work as expected.
Online forums (Stack Overflow and Reddit, among others) are always a source of knowledge, with the caveat that something may have been posted a few years ago and may not apply anymore. Always check before trying.
Finally, MongoDB University is a great place to keep your skills up to date and to learn about the latest features and additions: https://university.mongodb.com/.
