Perhaps you remember the feeling and perhaps that you had fun buzzy conversations? That would mean those were the important parts, not the actual substance of the essay. Nothing wrong with that.
Perhaps you changed a bit as a result, even if you can't remember precisely what spurred the change. Nothing wrong with that either.
And I bet there are books you read back then chunks of which you remember (and other parts that are gone). Probably you remember the good parts.
I don't think your memory has necessarily gotten any worse -- if anything (given you're in your 40s by inference) it's probably gotten functionally better.
Cluetrain Manifesto? I can't remember anything at all.
edit: Maybe it's like The World Is Flat? I've heard that people who read the book when it came out thought it was incredible, preposterous, and amazing. But if people read it for the first time today, they're like, "why is this so special?" Perhaps it's difficult to remember what Cluetrain was about because the world became Cluetrain? Except judging from all the comments, I'm guessing a lot of people were disappointed that it didn't. So I don't know.
I saw the headline, and struggled to remember any details of the book, and ended up having to read a summary of it. I only remember reading the book and talking about it with coworkers during the dot-com craze.