To me, the biggest problem with Clippy was that it was so damn patronising. As if it knew what you were wanting to do, and just had to help you. I did have it running, mainly because the animations kept me amused ( I am a simple person really ), not for the advice or comments, which I turned off.
And, as @Ben says, it interrupted you doing x to tell you it looked like you were doing x and would you like some help. It should be requested, not imposed. It always seemed to behave like a super-geek, who was always hovering around, telling you what you should be doing.
Finally, it was less clever than it seemed to be. So having pretended to know more than you, it often seems to get the wrong type of answers, because it had no more information that the help system Siri is a novelty, and is significantly better than Clippy, I gather, as it should be so many years later. But I think that Siri will also be ignored in a year or so, after the novelty has worn off.