While I agree with the sentiment you're expressing here on some levels (people don't like it when you insult things they like, as they feel they need to defend their choice or themselves because of it), I don't think that quite applies here.
This is a case where, although the ultimate complaint of the author is the behavior of the product (which is documented, but un-intuitive in nature unless you've read up on the issue), it's the way in which he chose to frame the problem that is getting people upset.
This is a known issue, even if it seems like a completely poor design decision. The issue I think most people here are taking is that because the author did almost no research on the topic, he got himself into a problem, and is trying to blame it on Mongo.
I don't think my GP was complaining about people saying the OP was wrong, but the vitriol associated with it. This article has prompted some of the ugliest comments I've seen on HN, even worse than Apple/Google crap articles.
Telling somebody they are wrong is one thing, calling them moronic or stupid is quite another.
I think this is an evolution of the language wars wherein immature developers align themselves with a technology and mix up criticisms of the technology with criticisms of themselves. This seems to be part of the need humans have to be part of a community.
1. Immature in this context has nothing to do with age. Rather, it is an attitude that shows when any developer has not experienced and internalized enough technology to realize every single technology has fundamental problems, sucks in some way, yet is still usually pretty amazing nonetheless, especially within the context of its creation.