I used to write my fics in Notepad (because this was the only way I knew how to do TXT files, way back in the day when I had computers that came only with Microsoft Works -- which saved word processing documents as WPS files... which weren't supported by FFNet), so while I knew how to start new paragraphs and had a decent grasp on capitalization and punctuation... my spelling was crap. Seriously. It was.
Also, because I didn't know how much I was actually writing, my stuff usually came out as one- or two-page POSes that were done in about an hour sitting in front of the computer. Some of my pieces today still are one- or two-page POSes, but I tended to be slightly less descriptive OR overly descriptive to the point where I'd mention irrelevant details.
Which brings up my most major problem: I HAD MARY SUES. Okay, yes, I admit it. I had Mary Sues. I wrote so many it's not even funny. Some of them were inserts of my friends. Some of them were me with prettier hair (oh, my love for purple hair back then) and the pants of my favorite character at the time. (There was one for Ash, one for James, one for Tracey, and somehow, Bill was a pedophile. *headdesk*) Some even had uber special POWERS -- mostly, psychic abilities and uncanny knacks for screwing with reality. It was not a proud era of my life, but at least I know now what I didn't know then.
Storylines tended to be hit and miss. Sometimes, I came up with interesting storylines (like a very early work of mine where Ash's Pikachu turned out to be the reincarnation of his father... which is a storyline I still like, even though I don't like any of the rest of the work and thank the gods that people actually took it as a humor fic), but most of the time, I liked to have my characters angst. I still do, but back then, I had very little grasp of key concepts like "plausibility" and "in-character." I won't go into examples because a lot of them are exceedingly painful to remember because they're so outlandish they're nearly comedies. I really didn't know how to do darkfic as a teenager. I really didn't. And yet, I tried and tried again to the point where most of my fics were pretty much that. Or crappy parodies and script fics. Okay, I didn't know how to do any genre decently.