It really is very subjective to the reviewer as to how much weight he wants to put on different aspects of the fanedit.
Compared to the orginal (better or worse), technical stuff, plot holes, editing choices, expectations, etc...
But I think it starts off with whether I even liked the original.
If I hated the original, it's very unlikely that I'd watch a fanedit of it.
Personally, I don't see how Adywan's SW gets such massive attention.
I like star wars, but I'm not THAT hard core about it.
Besides, if memory serves, Darth Editous' version seems to have the best video quality of all the EP IV's that I've seen.
Perhaps I even enjoyed that more than Adywan's.
But overall, I guess I start with how entertained I was and work from there.
Detracting for niggles and adding for positives.
For example, I probably would have given Azazel an 8 for entertainment, as I enjoyed the original more, but the editing was so good, the menus were amazing, and although it didn't have that extra twist ending that I love, it met expectations as the whole point was to make a straight up thriller. So I gave it a 10.
On the other hand, I probably would have given the Mist a 9 or a 10 based on entertainment but the video quality wasn't great so took off a point or 2.
So what's my point?
Erm... I guess... Kids, don't watch edits of movies that you hate.