I didn't have any problems with KOTB even at 11 years old. If you read the Moldvay B/X rulebook before reading the module (recommended) then it makes sense.
No, it doesn't. The monsters don't have believable motivation. The keep isn't in believable danger. That much I figured out immediately.
And later I started wondering why people motivated by treasure didn't rob the keep instead of the monsters, since the keep had more good stuff. So then I started wondering what motivation 'lawful' people had in this situation. My DM mentor revealed that his group (somewhat older) had actually played chaotic and robbed the keep. That was funny to a 12 year old, but it wasn't what I wanted from an adventure.
I also noted other big problems with the module as I was running it. There were these different humanoid tribes, but they were basically the same. You'd go into a cave, trigger (or rarely not) the alarm system, and then the whole tribe would come piling down on and there'd be this massive slugfest between all these humanoids and the PC's. There wasn't much tactics to it. There wasn't any space much for tactics beyond not getting surrounded. The ground was flat. The distances were short. You'd just roll dice. And roll. And roll. Early on there was a lot of random unavoidable death, particularly among the classes that couldn't wear heavy armor. They'd take javelins or arrows or something or get outflanked, and just die. But within about two forays, most everyone left alive, was in plate mail with shield +1's, and then the monsters really didn't stand a chance. And then it was just a matter of rolling dice. And rolling. And rolling. It was all rather repetitive. And it got me wondering, why in the world didn't the whole valley just come piling down on the PC's as soon as they were spotted. Like, if this was supposed to be some sort of army, if they are really working together - surely they could put aside their hatred of each other enough to blow a trumpet or something and send the whole 110 or so humanoids in the caves hurtling down on the PC's. I mean, yeah, they don't like each other, but even a chaotic can figure out 'the enemy of my enemy', right? Especially with these priests up there. And if they aren't working together, why are they even here? Why wait around near their mortal enemies, in danger of the keep at all times, just so some adventurers can come along and take their stuff?
I tried 'fixing it' as a kid, but I couldn't figure out how. I figured there had to be an army or evil mustering somewhere, nearby, but I couldn't figure out how to express that in a way that wasn't repetitive - you know, lots and lots of barracks filled with more and more humanoids. It was the caves problem all over again.
It was very frustrating as a young DM. I was trying to figure out how to write an adventure of my own, and all I had was this lame piece of crap as a template. It wasn't creative it all. It made me think that the way to write an adventure was to just cram a bunch of unrelated monsters together on a map. It was a terrible 'introductory' adventure. Reading it again as an adult, I'm struck by how advanced in skills a DM has to be to run it really well and just how badly I'd ran it as a kid. Even my 17 year old self's reattempt, though it had some merit, still failed the real test. Imperialus's 50 pages of notes is on the mark - more content than is actually in the module. It would take me probably 50 hours to get B2 ready to run.
CM3 Saber River, ran by mentor, on the other hand was a revelation.