One of my favorite aspects of this hobby being able to take these taboos, making a good video out of them, and completely shoving them in the naysayer's faces.

I know there's the obvious tech taboos like "don't encode in wmv", "don't use mp3s in premiere", which are plentiful, but I would rather focus on the taboos that focus on the creative aspect.
Probably the biggest taboo is "Don't make a crappy video", but one man's trash can be another man's treasure.
Here are the ones I remember:
Popular music + Populpar anime = Bad: This one probably still stands today, but I remember back then, people would make amvs with the popular song which was literally just background noise in the video since the video had little to no editing.
Using more unknown sources = Better Video, Popular source = Bad Video: A good or bad video can be done with any source. Unknown shit is still shit.
DB/Z/GT = Bad: This was the overabundance of (bad) DBZ videos made and especially with the series' reputation, many people who never watched the show would automatically deem anything with DBZ bad. I would like to say that I (and a few others) will continue stomping out this taboo as much as we can.

Linkin Park = Bad: Linkin Park is the most overused band for videos and their songs are always on the emo/angsty side which obviously led to this one. The irony is that all of the Linkin Park hate back in the day turned me into a fan.
Linkin Ball Z: The result of the above 2 combined. I think this died out in 2002

Song from the anime + anime = Bad: This by far is the stupidest taboo I've ever heard. I know it's a preference for some people not to use the same songs from the anime since it just feels like watching the anime itself. I remember one guy on old phorum (remember that?) was so adamant on this that he would say you deserve 0 originality. The greatest thing was when he was pwned by many other editors, even a few old school ones.

Effects = Bad: This one's still going on today, but if you're creative enough, you can use them to enhance your video, yadda, yadda, yadda.
I know there's tons more, but I can't think of any right now since I have to get back to work. Feel free to add more!