That's about it; also, don't approve of every review thrown your way, either. Take from the reviews what is actually correct, or what might make it easier to read / fix it up a bit.
Don't discount reviews, either. You post your story on a forum to get a full spectrum of feedback from objective reviewers, not just bits and pieces
you believe to be true. The only way a review can be absolutely false is if it either can be disproven by fact (read: someone is telling you to break the rules of grammar) or can't be fit into your story after all (like a suggestion you can't use because it you aren't able to work with it after all -- for example, a plot point suggestion that you can't meet because your plans go in a completely different direction that you're going to explain thoroughly later on).
Instead of calling a review correct or incorrect and instead of passing judgement on whether or not you actually pay attention to a review because you wish to "approve" of it, think about what's being said for awhile and try to understand
why a reviewer said it. All a review is is your story in someone else's eyes. If they spot what they believe to be a hole in your thinking, it's not necessarily outright incorrect. It's simply the way someone without author's bias sees your work.
Beyond that, lucariojon, aside from what others have said, my advice would be to simply write and post. Everyone's first fanfiction is at most mediocre, but that's perfectly okay. Allow the community to help you find ways to improve your style. It's hard to give you advice on how to weave a great fanfiction without having an understanding about what you already know from past writing experiences (roleplaying and original fiction included). Otherwise, we may accidentally give you advice that doesn't actually help you. The only way to gauge your skill is by having you show us what you can do and working with you from there.
Otherwise, take lessons from the community. It's your best tool to learn. Read a lot of fanfiction and a lot of reviews -- both the long ones, the short ones, the good ones, and the bad ones. The best way to understand how fanfiction works is by reading a lot of it. That way, you can see the mistakes other people have made, the tips reviewers have given writers, what plots have been done to death, and so on and so forth.
Read discussion threads and get a handle on who your potential audience is. The way to get a popular fanfiction is to appeal to the likes of the community. If the members of the community want something fresh and new, give them something fresh and new, not a new trainer story like everyone else's. However, if the members of the community lap new trainer stories up like honey laced with cocaine (and there are some communities like this -- Fanfiction.net, for example), give them a new trainer story. This isn't necessarily the way to a good fanfiction, but the more views and feedback you get, the more likely you'll net some good advice that will help you refine your style and prepare yourself for some serious work. Not to mention that nowadays, there seems to be some confusion between what's quality writing and what's popular. Or, long story short, paying attention to what your readers want in terms of genre allows you to experiment and figure out what works and what doesn't without having a dead channel of feedback, if that makes sense.
I guess what I'm saying is:
1. Just post
something, and we'll be able to help you figure out where your strengths and weaknesses already lie, which in turn will help you figure out how to write great fanfiction. Granted, this means your first one may or may not be crap, but that's an experience everyone deals with.
2. If you're not willing to post without getting advice first (which is perfectly understandable because it
is rather intimidating sometimes), you've already got pages upon pages of great advice in the forum. The best way to learn is to get examples, and the best way to get examples is to see the information trade in action. Use everyone else as your way of figuring out what to do and what to avoid. It's a great way to be lazy and learn before you get chewed on.
Good luck with your first one. I'm looking forward to it.
