with MongoDB (and I assume other NoSQL database APIs worth their salt) the ways of querying the database are much more simplistic than SQL. There is no tedious SQL queries to generate and such. For instance take this from mongodb-csharp:
using MongoDB.Driver;
Mongo db = new Mongo();
db.Connect(); //Connect to localhost on the default port.
Document query = new Document();
query["field1"] = 10;
Document result = db["tests"]["reads"].FindOne(query);
db.Disconnect();
How could an ORM even simplify that? Is an ORM or other "database abstraction device" required on top of a decent NoSQL API?
Well, yes, Object-Relational mappers are redundant with MongoDB because MongoDB isn't a relational database, it's a Document-Oriented database.
So instead of SQL, you write queries in JSON. Unless you really, really want to write raw JSON, as opposed to, say, Linq, then you're still going to want to use a mapper. And if you don't want to create coupling against MongoDB itself, then you don't want to pass actual Document objects around, you want to map them to real POCOs.
The mapping is much easier with a document-oriented DB like MongoDB, because you have nested documents instead of relations, but that doesn't mean it goes away completely. It just means you've substituted one type of "impedance mismatch" for a different, slightly-less-dramatic mismatch.