While I think the criticism of Cook's L&Ls was a bit overdone, I have to admit that they were kinda meh. It really stood out to me when they released a
collection of all the Legends & Lore columns at the end of 2011. Mearls' articles are a nice length, almost always at least 3 1/2 columns (in the pdf), and explore an issue historically, bringing up advantages and disadvantages, and suggest innovations. They generated a lot of discussions on the various boards, and while not everyone agreed with some of Mearls' ideas, they hardly ever had an issue with the column itself.
IMO, Cook started off pretty well, passive perception gaffe aside. But rather than the tight, structured articles that Mearls wrote, his tended to be rather stream of consciousness. He had an idea, he had a point, but it seemed like he was always floating around it. Easy to see in retrospect that he was talking about what they were doing in D&DN, but I guess he was being deliberately vague since it was before the announcement. Some articles just seemed like rehashes of what Mearls had written about earlier (e.g., the climb skill article, the article on class complexity and customization). From November on, it almost seems like he's phoning them in. They are barely filling 3 columns of text, and the content is basically "Some people might say X. Other people might say Y. I think many DMs do Z." There was no meat, rarely any ideas to really talk about. I expected the training wheels to come off with the official announcement of D&DN. But holy crap, after the announcement L&L got even less informative! The first two post-announcement Cook articles are contentless marketing pablum. The next three are general retrospectives on past editions of D&D -- nothing about what they were thinking of putting in the next edition, no real talk about design at all. The last one was the famous "gender-based limits on ability scores" one. And you know, my problem with that was not that Cook made a joke poll. If anything it just showed that the polls were just for generating discussion, not really being taking seriously as surveys (something Mearls explicitly noted in one of his early L&Ls). But that whole column could be summed up like this: "There are rules, like X, Y, and Z that will definitely not be in the next edition. There will also be rules from old editions. We want to hear from you about what you want. Here's a poll! But it's just for





and giggles, so it won't be used to get an idea of what you really want." Meaningless polls are fine when the article has some real content. But when the article content is basically "We want to hear what you have to say," and the poll is meaningless, well that's a metaphorical slap in the face. I felt like I wasted my time reading that article.
His next article is on high-level play. It's short, and Cook just riffs on the subject for a few paragraphs. At the time he wrote this,
they weren't even looking at high-level play for D&DN.* There's no design meat on the bone, no ideas. Plus, he basically denies the experiences of a whole bunch of players. His last article is IMO his best, as it actually talks about what they are doing with D&DN. It actually suggests ways of doing things. Of course he then equates playing Vancian spellcasters with "smart play". In one fell swoop, he puts down (unintendedly, I think) players who don't play Wizards
and players who don't want to play Vancian casters.
After that Mearls took over again, and IMO they immediately took a turn for the better. He talked about specific design goals and design ideas (speedy play, turn mechanics, save mechanics).
Even if one weren't predisposed to distrust Cook (and I certain wasn't), I think virtually everyone would agree that Rule-of-Three was always the better source of information, both under Rich Baker and Rodney Thompson. Thompson is at least a developer on the design team, and Baker was only peripherally involved with 5e. But why were their columns so much more informative, so much more interesting to read than that of the
Design Team Lead?
*I base this on the fact that as late as the PAX seminar Mearls said they were at best 20% along the way. I doubt they were far enough along to really look at high level play back in February.