One bit of advice most frequently heard among blog comments, generally in the vicinity of blog posts about blogging itself, is to Write What You Want, and from there Write What You Know. Which may be all well and good, but this runs smack into the adage of Knowing Your Audience: if your Audience doesn't Know what You Know, then whatever has been Written is likely to go unRead.
Then again, I never promised anything other than two pings on your chosen RSS feed per week, whether or not these pings are of any use. So it goes.
If forced to identify myself in the overall hierarchy of fandom, I would place my name firmly in the nebulous mass of the fanfic writers. Considering the reputation fanfic has among the other parts of fandom, and the derision that fanfic writers themselves (I do not say "ourselves" because I've never done it personally; I am in the subsection of lazy fanfic writers, far too lazy to bother commenting on other people's fanfics) heap upon fanfics which do not meet some arbitrary standard of plot, it is perhaps not surprising that I am quite comfortable by now with my reputation for having no taste whatsoever.
Even worse, I happily commit the sin of creating new characters for use in my own fanfics. These are labelled "original characters", which I suspect provides for a nice shorthand label of "OC", like "AU" or "gen". This is odd, since these characters are not exactly original, save in the sense that they are not native to the canon.
Some of the time, this is because I needed a personality type that is not available in the series itself: Card Captor Sakura was lacking in the deadpan Spineless Harem Comedy Character type, which led to ten-year old Ichiro Onosaka and his unrequited crush on Tomoyo Daidouji. Mostly, however, I just thought that it would be so cool to be a character in that world, and so I create Significant or Powered characters to live vicariously through. Self-inserts, essentially, except with different names, personalities, and pretty much everything except wish-fulfilment.
Such characters, and their authors, are often claimed to be the scum of the Internet, implying that we are worse than 4chan, which has to be an incredible achievement in its own right. It is seen to be in bad taste; since I am a primary fan of Moe Fanservice Anime, though, I have no shame.
And then there are the Mary Sues. This term has been bandied about the Internet for so long that it should need no explanation, but since I'd rather not simply assume, we shall work on the definition that a Mary Sue is a character who is so powerful, so perfect, and so well-loved, that she (or he, for that matter; such are called Marty Stus, or Gary Stus, or some such) overshadows the canon characters. The astute reader may immediately spot the problem with this definition (namely, the sheer subjectivity of every important term), which is probably why the term "Mary Sue" has been argued about for as long as it has existed in the fandom consciousness.
As an example, let me show you them introduce to you one of my favourite original characters to write, a Mary Sue from the marginally-post-StrikerS era of Mahou Shoujo Lyrical Nanoha, Lumina Celeste.