Of course, you can always jump on Kalgan's posts about warriors being "tank/dps" hybrid as proof that warriors are hybrids. In turn, relate this to priests as the "healing/dps" hybrids. After all, he really should know what he's talking about considering he is "the Lead Designer responsible for game systems – character classes..."
Even I've pushed the warriors-and-priests-are-hyrbids argument when it suited me. After all, what's the difference between shaman being "healing/dps" hybrids and priests being "healing/dps" hybrids?
So, while technically priests and warriors can be considered hybrids because of their ability to fill different roles, if we wanted to apply the definition that way, then you suddenly turn to other classes as well. Suddenly other classes are filling in other roles. Warlocks tanking on the Twin Emperors, mages tanking Krosh Firehand, hunter pets tanking Anub'rekhan; but let's be honest, no one is going so far to say these classes are hybrids (and certainly not me.)
So while it's nice to say a class that fills more than one role is considered a hybrid, the reality is that it's wrong.
In fact, to look at this properly, a hybrid isn't a class that can fill in multiple roles, a hybrid is a class that isn't defined by a single, traditional role. Warriors are tanks. Priests are healers. This is their primary role. This is their primary function. Sure, they can fill in other roles as needed, but the developers made them tanks and healers first and foremost.
Indeed, there have been countless posts concerning priests and warrior's roles and their primary roles indeed being the pinnacle of that role. Developers want warriors to be the best tanks, and if warriors aren't generally the best tanks, they will work to ensure this. Priests should be the best healers, and developers see any lack in this area as a problem. Sure, these classes can also perform other roles, but that doesn't mean these classes are intended to be any worse in their primarily intended role.
Traditional hybrids, however, do not get the same treatment. Shamans, paladins, and druids are all intended to be balanced around the idea that they can fill in different roles. We don't have a primary role or a primary purpose. We are hybrids because of that very nature.
And that's the difference, and that's why warriors and priests aren't hybrids in the intended sense. Sure, they might be able to fill different roles, but they have a primary purpose.
Of course, that doesn't mean people won't disagree. But I wonder how they'd feel classifying warlocks as tanks. After all, they also tank on Leotheras.
During the day, Jason Lotito browses the WoW forums. But by night, he takes the form of Endure, a level 70 paladin, and faces off against the toughest bosses Blizzard has to offer with his guild at his side. He's previously played a shaman to 60 and raided Horde for a while, and is currently leveling a druid just to see what all the fuss is about.