I read Susan Cain's book in my early 20s, and it really was a relief of sorts - it took away some of the "what is wrong with me" feeling. However, even then, I found myself annoyed by a fair amount of what she wrote. I took objection to the portrayal of extroverts as shallow and annoying. I didn't see extroverts that way at all. Extroverts didn't/don't annoy me, rather, the social expectation to be more extroverted can frustrate me.
I also didn't like the special snowflake angle, especially when it suggests asking for over-accommodating to the point of being as obnoxious as some extrovert who won't shut up. One suggestion was something like wearing a sign around your neck at work so people know if you are open/closed for talking (similar to a sig you might put on a door). Like, it's not going to kill me if a co-worker talks to me as I pass by... It almost seems attention-seeking rather than setting boundaries.
Because her book is not very Jungian influenced, all different types of introverts and extroverts get lumped together. I suspect she is an INFJ 4, and her description of extroverts all sound like extreme ESTP 3s or 7s. There is some association with introverts being more cautious or less risk-taking - possibly not true for many IxxPs, as I'd argue that cautiousness is more related to being a Pi-dom. Because of this, a fair amount of ExxJs strike me as rather cautious or more adverse to certain kinds of risks. And of course, plenty of extroverts are philosophical, intelligent, creative, have depth, etc. I really think it is this bias that confuses people, along with the absurd idea that extroverts wouldn't ever "recharge" by being alone, given it's a very human thing to need rest and quiet when overtaxed.
IMO, [MENTION=14857]labyrinthine[/MENTION] 's explanation of I/E differences and spectrum to be much clearer. I also take into account the Jungian types and note a person's mental focus, which is often how to spot an ENxx type who otherwise may be incorrectly identified as an introvert. Still not a fan of hers, but I'll give credit to Vicky Jo for simply explaining how to spot introverted vs extroverted mentality as a focus of the thoughts, not necessarily whether the thoughts are vocalized or acted on.