This problem seems pretty universal to online rankings. Simply put, the biggest problem: if I don't really like or dislike something, what's the motivation for me to take the fraction of a second to give it a ranking?
Looking a bit deeper at the matter, as an illustrative aside (which I'll loop back to MD), I have a collection of several thousand LPs which I have cataloged on Discogs, and for the classical titles (which are about 2/3rds of my collection) I've entered my rankings on the 1-5 star scale they use. Like MD, a 0 is unranked - the lowest possible ranking is a 1. These rankings are public, so my rankings get averaged with everyone else's to determine the community rankings for each title, though I can see my individual rankings in my personal collection.
The problem with relying on the community averages comes in how someone decides what criteria to use for each ranking. I personally am using those for the quality of the release from an audiophile standpoint, so I can lay hands on the best recordings of any given work. A 5 is an exceptional performance, a high-quality recording, and an excellent pressing; 4 would be above average in at least two of those; etc. I only have one or two discs that I've ranked a 1 - why would I own bad records with bad music? - and as I've gotten more experienced, I've started searching out certain groups, labels, etc. to improve my chances of getting 4 or 5 star releases when I go to the local record store. This means my average ranking is up to about a 4.25 at this point. Am I ranking too high, or is there simply a selection effect happening?
Many people seem to give rankings based on whether they like the album or not. So a disc I might think is a 5 could have an average of 3 because most people don't care for the music, or a disc with a 4.5 average might be a 3 in my list because the recording quality and pressing are sub-par, even though the music itself is excellent. This gets amplified a lot for something like classic rock - which I don't rank - to the point where just about every significant album of the '70's has above a 4.5 star average. (I'm sorry, but Atom Heart Mother and Presence just aren't that great.)
Coming back to MD, am I (and a thousand other people) ranking a work based on my response to it - this manga has "wholesome fluff" or "compelling characters" or "BOOBA!" so take my ratings points! - or on the art, or on the translation, or...? So the problem becomes a matter of a single dimension being used a number of different ways by people, compounded by a reluctance to rate something 'in the middle' (note the tendency to skew toward the outsides of the range rather than the middle on many titles) - hyperbole made manifest, in a way.
TL;DR - if you're actually thinking about the ratings, you're not doing it wrong. But don't expect everyone to follow suit.