Good:
Videos that completely rewrite characters in new ways, that are internally consistent.
Videos that are really good at telling stories, especially if they are side characters that don't get overly used much.
Videos that are such a perfect match of concept and execution that I can't listen to that same song again without mentally thinking about the video.
A lot of times I find that I like the raw enthusiasm of newbie videos compared to the polished perfection of some established editors.
Videos that are just plain fun and leave me feeling happy.
Videos that are visually stunning, and artistic in imaginative ways.
Examples of good:
Wasteland - Otohiko One of my favorites.
Another Accidental Love Story - Bakadeshi Lighthearted and uses entirely secondary characters to great effect.
Blind Mans tale - Otohiko (again). Inventive and dramatic.
Kyoto Rain - McKeed. One of the first videos I ever saw. It's kind of long, but still fits the mood of the show perfectly.
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Bad: =(
Nasty Hentai videos (lighthearted ones can be fun.. but it's rare)
Videos that give me a seizure and make me feel like my eyes are being scraped by a file over and over.
Videos that are basically a scrambled egg. (stuff tossed together and baked)
Examples of bad:
sorry, you'll have to find out the hard way.