In the discussion of the previous article posted a couple of days ago, a lot of people complained that he initially didn't provide source code of the tests or go into details about the Mongo's configuration. Some felt that as someone who's worked with internals of RDBMS's, it wouldn't be fair to compare something he knows intimately with something he just learned and didn't spend any time optimizing.

After he posted the code, others complained that the scenarios didn't have enough multi-table joins (which Mongo would represent as nested objects and, at least in the commenters' opinions, would probably do better than it did). There was also a lot of more detailed technical discussions but I won't try to summarize them:

http://news.ycombinator.com/item?id=2161753

(Note: these opinions aren't my own, just what I got out of the discussion.)