I'll try to explain my ratings here. These will likely change over time as I watch more anime and get a better sense for what I like and dislike. Also, none of these ratings are absolute. Think of them like an approximation, what may be a 7 for me could be an 8 or a 6 for you, depending on your preferences. There are definitely exceptions and strange reasons why I rate a certain anime a certain number. Just ask and I'll gladly tell you what I liked and disliked about any series.
10 - Amazing. These are personal favorites of mine. They're completely solid shows with only very minor, easily forgiven flaws.
9 - The only big difference between 9's and 10's is that 10's are personal favorites that I've likely rewatched at least once. 9's are completely solid shows that excel in every way, with very few minor flaws.
8 - A very good, solid show. No major flaws to speak of. I really enjoy these shows. The might not have the one thing that would have pushed them to a 9, but they're still excellent shows overall.
7 - A good show. Maybe a little shaky in some areas, but strong in others. These aren't quite solid enough to earn 8's, but are good nonetheless.
6 - Decent. Passable. There is nothing really horrible, but nothing of exceptional quality either. Shows that have average premises, and execute them decently go here. This is NOT a bad show, it was watchable, it's just not as good as it could be.
5 - Not good enough. These are not quite up to par, they might be average shows that are executed averagely. Nothing of note. Or maybe they were awful, but had a few redeeming qualities. Either way, not recommendable.
4 - Urgh. These are not very watchable. Maybe a pain to sit through. Maybe I just don't like them. They're bad, with maybe one decent/good quality.
3 - No. Go away. We don't need these. There is virtually nothing good. Maybe a few average qualities, but otherwise. No.
2 - Bottom of the barrel gunk. There is nothing good about these. Everything is subpar, uninspired, lifeless, garbage.
1 - Like the difference between 9 and 10, the difference between 2 and 1 is mostly personal preference. I despise this so much that I can't think of anything worse. In a way, the fact that I hate it so much means that it might very well be a "better" score than a 2, since it at least sparked a reaction.
Also, in terms of how easily I'd recommend a title to someone:
1-5 = Not recommendable for the most part
6-10 = Recommendable for the most part