Thanks for the question.
I cannot completely assure you that people aren't learning MongoDB at least in part because they don't want to learn or use SQL. MongoDB has a fairly intuitive query language, and it's a small reason why people gravitate toward it.
As with any database, success with MongoDB does depend on being able to think critically about how you're going to use your data. MongoDB supports rich, dynamic data structures, and you can change them on the fly without having to issue ALTER TABLE statements, but this does not mean that you can get away with sloppy data modeling.
We've seen the same problem of people wanting to use ORMs to hide the realities of the database, but this doesn't work for non-trivial applications, just as it doesn't for SQL databases.
As for use cases, I'd say that MongoDB is ideal for these situations:
1. When the application's data is inherently unstructured. Think products in an e-commerce site. Each product can have an arbitrary set of attributes. MongoDB documents make this pretty easy to model.
2. Rich data models that don't require joins. You'll often see relational schemas that break a single "object" into a dozen different tables. If the object in question has to be constructed using a SQL join every time it's displayed, and if there's no ancillary benefit to having the data modeled in this way, then there's a lot of unnecessary added complexity there. Consider a page in a content management system. Why does each individual element need to be in separate record or table? A MongoDB document can typically store all these elements in a structured way while still facilitating sophisticated queries over them. This has the added benefit of providing good locality.
3. Analytics. I won't go into detail now, but there are certain types of analytics applications (think website activity tracking) that MongoDB has been optimized for.
4. High availability. MongoDB's replication system provides automated failover.
5. Sharding. If you have a lot of data but want to run on commodity hardware, MongoDB 's sharding can be quite compelling.
Hope that helps!