In essence, a huntard is a hunter who does not know how to play his class well. This stereotype grew to the point many players were reflexively skeptical or rejected out of hand any new or inexperienced player using the class, regardless of that player's actual competence. While slang to describe badly-played classes is common, hunters have this stigma above all others for a number of reasons.
They can easily dump aggro (more than likely onto the healer when in a group), previously had great flexibility with weapons, and can avoid fighting dangerous close range situations. Huntards are especially disliked in low-level PUGs, where inexperience with managing pet aggro and movement - versus warlocks, who despite having easily identifiable threat-pulling pets had no threat dump and presumably played more cautiously - can pull threat from a tank and cause annoying and expensive wipes; the ease of questing solo, even in early vanilla content, meant many hunters had no experience running content with groups even at higher levels.
Huntards were also typified as rolling Need on items and equipment inappropriate or badly itemized to their class, creating resentment among competitors in groups. The restrictions on loot rolls in higher level content and complete removal of melee weapons and StatSticks for the class have curbed this somewhat, at least in endgame content. A huntard's combat incompetence was also implictly associated with a self-centered, overconfident social attitude, such as claiming pet tanking to always be a viable option and evading blame for any mistakes made that endangered or wiped the group.
On the other hand, the term can be used with more neutral and even endearing tones for one's own hunter, particularly after many of the worst traits became associated with the new death knight class during Wrath of the Lich King.