I would say mostly breaking (or more bending) the plausibility of realistic speech, realistic reaction (physical or emotional), and telling instead of showing, or using too many nods and shrugs and adverbs (he said excitedly). It is relying too heavily on some same "formula" for description or exposition, to the point it becomes noticeable.
I should note that most of their critics have not made tens of millions of dollars in their writing!
As a result, I feel it is much more important to turn your question around. Despite their shortcomings (not failures, since they are published), what are they doing right? Why do readers overlook these shortcomings and buy their books? Why do producers overlook them and turn their work into blockbuster movies?
I think the answer is, these imperfect writers concoct compelling and original plots and settings, at least for their intended audience. That is what the movie producers buy, that is what the readers are looking forward to.
The lesson from this should not be "what pitfalls to avoid", because clearly you don't have to avoid them to get published! The lesson should be "What can I do right so I don't have to be a perfect writer, and can get published with all the imperfections of a strong effort?"
The answer to that is much harder work than becoming a good technical writer: You have to invent a good original story with something about it people (most of them) have not fully imagined before. Before JK Rowling, I would not have thought of a Wizard's school that would appeal to a children's audience. Before Dan Brown, I would not have thought of Christian artifacts, statues, buildings and manuscripts of having hidden clues to a major secret being covered up by the Vatican. Both of those are genius ideas, superb stories imperfectly realized.
So if you had to choose, which would you rather have produced? A blockbuster novel riddled with examples of cringe-worthy writing, or a technically perfect tome that sells a few thousand copies?
It might be nice to have both an awesome story with a compelling plot, and expert writing, but if you have the story, then "decent" or "serviceable" writing is enough to get it published. Just look at JK Rowling or Dan Brown, both have risen from the middle class to the 0.1% on the strength of original (and captivating) imagination throughout their first efforts.
P.S. As for their critics from the ranks of professional best selling authors: JK Rowling and Dan Brown are #1 and #2 in net worth of ALL authors with about $700M each, and this is after JK Rowling has given over $100M to charitable causes. Stephen King is fifth (400M). Rowling has sold 450 million books, Stephen King 300 million. Tolkien and Dan Brown are tied at 200 million.
P.P.S. A late additional note on the idea that great marketing can create a blockbuster. I am the sole author of dozens of best selling ads; brochures, coupons, ten page direct mail letters, full page newspaper and magazine ads that ran for years, getting two to four times the national average response rates. I know something about marketing. Check out this short Wiki link on Diffusion of Innovation, particularly on that page The Adopter Categories which are shown in order.
The summary argument is that most people in a potential audience, about 85%, will not buy a new product without a trusted recommendation from somebody else. Ads and paid celebrity endorsement are selling primarily to a small minority percentage of the audience, they only work for the rest if they get a trusted recommendation from a source they consider unbiased, which generally requires the source to be an unpaid friend or other social contact, or a professional critic that they believe does not sell their good recommendations.
Thus, it is effectively impossible for Marketing to create a best seller out of a book that is not very entertaining to actual readers, even excellent ad writing and production isn't going to cover the gap of "word of mouth" marketing. Innovators (which would include critics) and perhaps very Early Adopters may buy the book because of the ad, they are willing to take the risk on a new author or new style, they can afford to buy a bad book and laugh it off and drop it in the trash. But most of us are wary of ads and stingy with our time and money. So we count on first readers (including critics whose reputation is at stake) saying, "Buy this book," or "I just read a great new author." Marketing can light a fuse, but the fuse fizzles out if the First Readers are disappointed, or start a negative buzz.
Marketing is similar to a free video that goes viral on social media and gets 100K views. Even though they are free, 99.9% of videos never get there, because they evoke no strong visceral reaction (positive would be laughter or surprised astonishment or disbelief at some feat, negative would be horror or anger or empathetic pain). Without that strong reaction few people feel compelled to share it, and the virus fizzles.
A heavily marketed book works the same way, but the threshold of "visceral reaction" before passing it along is much higher for the first readers than even a free video, because now money is at stake, and we don't tell our friends (or fans) to spend money unless we are pretty sure they will be glad they did. We don't want to be known for making lame recommendations.
On the flip side, even if we love our friends (or celebrities) we ignore the buying recommendations of those that have fizzled out on us too often.
The route to a blockbuster is a chain reaction of free word of mouth when most readers are "moved" in some way and believe their friends will be moved, too. Marketing is necessary but not enough on its own: Content must have delivered on the promise. It is "original" content in my view, else some previous published work would have evoked the visceral reactions needed to fuel the viral sharing and complete the chain reaction. Then the previous similar work would have dominated the market.
P.P.P.S. A note on "viral marketing" due to a comment below. It is possible an advertisement has all the qualities to go viral. We have all seen great commercials that made us laugh, or had a great jingle or dance, or a twist ending. It isn't easy to pull off, but it happens, the ad does something creative and memorable and worth passing along. So if it is easy to pass along we might do that, or at least mention it.
But ads are free, and it still doesn't make us buy the product being advertised. Sometimes we can't even remember the product being advertised! (Which brings up the conundrum: If an ad is memorable, but you cannot recall the brand it was selling, is it good entertainment but a bad ad that didn't do its job?)
A viral advertisement creates a lot of free air time, and that should increase exposure and sales to Innovators (the few that don't need a recommendation). The ad might prompt the rest of us to inquire about the product it is selling and seek a recommendation. But even a viral ad will not magically take away the need for a recommendation from the 85% that won't buy without a positive review. A viral ad (free, short and easy to share via a link to a video or image) will not result in blockbuster sales of a product people don't like.
That includes a book that is boring or not worth the money or time to read it. Especially if the Innovators that can afford the time and money to buy it with no recommendation then tell people it isn't as good as the ad makes it look. Bad word of mouth is passed along ten times more readily than good word of mouth.
That differential is because it is always worth saving your friends time and money, and complaint is your reward in a way, a bit of vengeance for feeling ripped off, and your friends (if you are not a constant complainer) will appreciate the heads up, even if they decide to spend the money anyway: Nobody blames you if you thought something was terrible and said so.
On the other hand, it is always a risk to recommend anything to your friends that will cost them time and money if they don't like it. It indicates you don't know their tastes very well, and it can feel to both of you like you have made a mistake and they would have been better off if you had just not said anything. So unless the enthusiasm is very high, most of us do exactly that: We don't volunteer information about "okay" products we tried, and if asked about something that did not produce a great experience we tend to tone down whatever enthusiasm we DO have and offer tepid endorsements. "I kind of liked it, but it may not be for everybody..."
Viral marketing is very effective if it advertises an equally good product. It can backfire if it advertises an imperfect product, the popularity of the ad can mean people rush to **dis-**endorse the product, to do their friends a favor by warning them about a rip-off. Thus, it is not a route to turning a mediocre book into a blockbuster.
Put another way, viral marketing can increase the amount of talk about your product, but you have no control over the content of that talk in texts, tweets, emails, reviews, blogs, comments and up/down votes on the sales venues. You may be amplifying harsh criticisms.