But why, then, did KotS get such a bad reaction? It can't just be all the issues around high(ish) level play, and scaling DCs, and the like, becaues none of those are in issue in a 1st level module. It seems that even at 1st level a lot of people found 4e unsatisfactory for "traditional" play. What were they experiencing that Mearls and Cordell missed?
To be honest I don't think the reaction was so bad. It got the Penny Arcade guys into 4e, particularly Mike Kruhulik, who previously had no experience with or interest in tabletop RPGs. Wil Wheaton liked it and used it for the campaign he ran for his sons. The Amazon reviews are quite positive. 20 five-star reviews, 25 four-star reviews, 20 three-star reviews, 8 two-star reviews, and 8 one-star reviews. The negative reviews either decry the physical product (flimsy) or desire more story or plot. A lot of the 5-star reviews strike me as unbridled enthusiasm ("We just got this can had one battle! Awesome! 5 stars!"), but the four star reviews largely echo my assessment earlier: solid, if uninspired.
It seems to me that a lot of the negative reaction to KotS falls into two types, which one can see on the Amazon reviews. 3e players who use it to whip 4e. I.e., "4e is all about combat. Look at KotS: you just move from one combat to another." And then 4e players who note that it has little in the way of story or narrative weight to its encounters -- it's just a largely linear string of battles until you get to the boss. In a sense, if you're sensitive to the "4e is just aping MMOs," then KotS is a poor module because it plays into that stereotype, without playing to the wholly non-MMO-like strengths of 4e.
However, it should be noted that such video-game like structure is in fact a legacy from traditional D&D itself. Many a player enjoyed TSR-D&D and 3e doing exactly that: have a decent background story for the setting and reason for adventuring, then have the PCs go door-to-door kicking evil butt. And 4e, at least initially, goes well with that playstyle thanks to its detailed combat engine and harder-to-kill characters. There are certainly lots of vocal critics of 4e who laugh and point at the module, and vocal 4e players who think it doesn't play to 4e's strengths. But IMO there are a lot of players who played the game that way and enjoyed it, and enjoyed KotS. I also suspect that many such 4e players also tend to be more accepting of 5e, because while it doesn't look much like 4e mechanically, it provides them the same kind of play they enjoyed in 4e. But I admit that this last part is me extrapolating from my own experience.
(As an aside, I think there are two flaws in KotS from a traditional dungeoncrawl point of view. One is that there are many Encounters in which battle to the death is the assumed outcome. It's fine if you want to play that way, and fine for a few encounters to be that way, but I think more non-combat alternatives should have been offered. Compare Keep on the Borderlands, with its suggestions of setting the monster tribes against each other. The other flaw is the dungeon is not
jaquayed enough.)