I still remember using Windos ME as a kid and loved it because it felt more modern than Win98. I still don't get what was so bad about it but then again even if I would know I would have no idea.
Apparently, according to Wikipedia: "Although Windows 9x features memory protection, it does not protect the first megabyte of memory from userland applications. This area of memory contains code critical to the functioning of the operating system, and by writing into this area of memory an application can crash or freeze the operating system. This was a source of instability as faulty applications could accidentally write into this region and with that halt the operating system"
As it was aimed exclusively at home users, various features aimed at enterprise (but often desirable to power users) were stripped out or broken.