I find the limitations of their query language interesting, no way to test for undefined properties in a collection (the client libraries just give up, and I do

I find the limitations of their query language interesting, no way to test for undefined properties in a collection (the client libraries just give up, and I don't see a workaround); and no way to match on inequality rather than equality. This exists in some other odd database engines, but it's frustrating in general use scenarios (where, in this case, I don't exactly see how performance is a limiting factor).