Reviewability is always going to vary from person to person depending on their personal bias. Whether it's genre, style, music, anime, etc...
Me personally, I don't really favor one over the other. Yeah, I'll get excited over a video using my favorite sources, but that doesn't mean I'll want to watch a ton of times. I prefer to watch videos I consider good regardless of source or style.
Some things I look for are:
If there is a story, does it make sense? If there is no story and it's a random mood video, is there something that makes it stand out from the others? If the video is effects for the sake of effects are they well implemented or cheap presets that are left unaltered. What about this video makes it stand out from the others that brings me to watch it over and over again? What sets it apart from the others? Can you see the editor's dedication and hard work or is it just something they threw together because it was pretty?
In other words, the videos I like to watch over and over again are videos that have some kind of interesting catch or are unique.
Call me weird but one of my favorite videos of all time is this:
(I would recommend watching it with subs xD)
When I first watched it I thought it was really odd and overall, just weird. But that's what I love about it. It's totally out there but you can totally tell just how much this editor loved their sources and had fun with them. The effects aren't the best ever but it's not your typical romantic video and I adore it :3
Well, honestly I think it's a subjective thing. Sometimes the review ability of a video are influenced by the songs and anime used in the video. If I like that anime and that song, I will re-watch the video better than if I don't like them.
I don't watch amvs all the time, so when I pull one up it's usually because something reminded of that particular vid, or because I'm looking for a vid that epitomizes whatever is on my mind (editing genre, anime focus, etc).
1) Triggers Something in that video stuck in my head, so other things remind me of it later and I go back and watch the vid again (cementing that 'thing' even deeper into my head). So for this sort of vid, the reviewability increases the more I rewatch it. The vid is usually unique. Either no other vids feature that key word, or they just aren't as good and thus don't stand out as the "prime example". EX: The word 'butterfly' automatically makes my nieces want to watch KentaroPJJ's Narutowned amvs because there's a sequence in one of them with a song with 'butterfly' in the lyrics. That's not the best track in the vid, and they don't know the song or the character it's set to, but it's the one that stuck deeply enough to strike up a craving any time the 'key word' is used. I have the same reaction myself when I run across the word 'defile' - I think about Zarxrax's "Mystery Yaoi Theater 3000" because of the line about Berserk.
2) Anime When I'm watching an anime or looking for vids to show someone who has just seen the anime for the first time, these are the vids I rewatch. I pick my favorite character vids because of the way they portray the characters, the scenes they use and the song. Then, among those favorites, I'll cut them down by throwing out the truly icky ones (capture & editing). I will rewatch an "average" vid forever if the better edited vids don't use the scene I want. The vids I keep and rewatch for years stand out as the "prime example" of that anime/character/pairing. These favorites rarely get replaced. EX: I've kept tons of Sailor Moon vids over the years, but the one I always rewatch before anything else is Dokidoki's "Senshi on Springer" because it captures the cracktastic quality of SM that I like so much.
3) Editing/Genre I only watch these vids if I'm showing someone the type of vids people make, or if I'm trying to get in the mood to edit myself. These vids either epitomize the genre, or have something cool to awe whoever is watching over my shoulder. I have at least one 'prime example' vid for whatever quirk/genre I'm trying to show off (or get into myself). The only one that changes all the time is effects - if I'm showing that off, I just grab whatever vid got best effects in the last vca. EX: If I'm showing someone lipsynch, I play Istiv's "Shounen Bushidou". If I'm in the mood to edit action, I watch Fluxmeister's "Eternal Damnation". If I'm in the mood to try an AU vid (or if I'm trying to show off AU type vids), I play ScorpionsUltd's "Whisper of the Beast". If I'm after fan-animated vids, I play Darci-San's "Swinging on a Star". If I'm looking for a dance vid, I play AD's "Do it right".
As for how important reviewabiity is...I think it only matters for the top 10%, and even there it's a joke. I lose vids and then it doesn't matter how much I would have rewatched it - unless I can find another copy, I'll never get to watch it again. Then there are always new vids coming out. If I find one to displace an old favorite, I may never watch that old one again. Just because I'm not watching it now doesn't mean it lacked reviewability. I might have reviewed that vid a thousand times over the years before it got bested by a new one. This is like originality. It's only original until someone else does it, too. I was a huge DBZ and YYH fan for years and had vids I reviewed constantly. Those vids no longer have reviewability for me because I'm not that interested anymore. But when I wanted them they were highly reviewable.
Unless we're talking about truly bad videos, really random generic crappers, where no one but the editor would want to rewatch them, then I don't think reviewability matters. If the vid is at least average then someone is going to find it highly reviewable. Maybe they'll lose interest right away, maybe they'll keep it for years. As long as there's at least one person who likes what the editor was trying to portray, and who can find the vid, then the video has reviewability.
Or are we talking about shelf life? If that's the real issue, then yes, as soon as something new comes out all the old vids go in the garbage. A new cute girl anime came out? Throw away the Haruhi vids. It's just like how Naruto replaced DBZ. All the kids move onto the new craze. We get the same thing with resolution and file types, when you're looking at the editing viewers. Who watches mpg anymore? Throw them all away. And divx? What's that? 360x240? Are you kidding? Even with an ipod, that's way too tiny to bother watching. And effects? Nothing that came out before Euphoria is worth keeping. Toss it all out. That's how shelf life works. If people made vids with an eye to shelf life, then they'd never make anything because within a few years even the best vids will be obsolete because screens will just get bigger, effects will get more complicated and thus cooler, and codecs will continue to advance. With each advancement, all the old crap gets dated if not tossed out altogether. The old people may continue watching that stuff, just like they continue buying vcrs, but the kids won't bother and they're the ones who really determine shelf life.