True. The author forgot to add "scalable" there -- but that is actually mean there.
I am studying Multi-Dimensional Indexing for more than 3y now and have implemented many of the state-of-the-art indexes. They are all not sufficient as especially MongoDB's implemention is insufficient in especially the scalability-domain.
Managing Highly-dimensional data and access to it: ...I'm thinking of e.g. geo/spatial data here. Where are the solutions out there?
http://www.mongodb.org/display/DOCS/Geospatial+Indexing