Voted the following 3 (in bold).
Slows down system; heavy use of system resources
Incompatible with other installed softs.
Bugs; "malfunctions" of various types - e.g. freezes, crashes, feature failures, etc.
You report bugs, but they are not fixed.
* Poor technical support.
Does not detect\block malware.
Cannot\does not remove malware properly\completely.
GUI is not user-friendly.
* Learning how to use\configure for maximum security is difficult\time intensive.
* Nagging doubts about ability to protect system.
(More details later)