Most fanfics aren't good, obviously. And even those ones that are in the 10% have many issues. With Ranma 1/2 the biggest issue seems to be keeping things humorous and not taking things so seriously. Ranma just isn't the type of guy to get really moody if following canon post manga and it is refreshing to see him being himself. The over the top-ness of this fic really makes it feel that it could happen. That it is plausible. THAT is what separates an enjoyable fic to one with over 1,500 favorites. Plus, having Ranma not overshadow (too much) the other characters as we see his weaknesses in action. (mostly his beat up everyone approach and failure with diplomacy)
I haven't even heard of Rosario + Vampire before this and it still flowed smoothly as I was introduced into this world as I read it. Tsukune is a great partner as a foil of Ranma and it is enjoyable how they play off each other.
The humor is there, the insane growing harems of unwanted girls is there, and last, but not least, the insanely destructive fights is there.
Ranma vastly overpowering everyone would usually be a bad thing but we are treated to a rare sight of the consequences of those actions for once in a fic breathing new life into this fandom. With it still going strong headed into its fourth year, I am glad I found this fic.