This is how my rating system works.
5 Stars: These are either:
1: Purely subjective. Basically the animes/manga I personally enjoyed the most and made me want to watch/read them more than once.
2: The anime/manga presented an accurate portrayal of the genre in my eyes and deserves a 5 even though I may not feel the "5 star vibe".
4.5 Stars: More or less the same as 5 star rated animes/manga but had some sort of flaw that shouldn't be overlooked. For example, an excellent anime but a terrible ending (Desert Punk), an anime with way too many fillers and/or inconsistencies I don't think were necessary in the plot that could have been easily avoided (Naruto*1), or sometimes going overboard with touchy moments that crossed the line, making it feel disingenuous (Clannad).
4 Stars: These are either:
1. Animes/manga that I think are worth watching/reading. Their story, animation/drawing and characters all score high. The story is balanced, it's likely to be enjoyable and won't be too ridiculous (Bleach*2). I'd say my 4 star animes/manga are something you watch for a satisfactory feel when you just can't find that 5 or 4.5 star anime.
2. Animes/manga that present their genres well. I may not have enjoyed them, but trying to judge from an objective p.o.v they deserve a good rating.
3.5 Stars: A fair amount of these are worth watching/reading but should be carefully chosen as a lot of them have one or several flaws that are hard to overlook. Some are more than ok but had some sort of fatal fuck-up (D.Gray-Man) or extreme touchy moments (like Fruits Basket).
3 Stars: This is close to the "don't bother stuff". These animes/manga are generally just versions of the animes/manga mentioned in 3.5 but with things you can't get past x2. Some of them can be good but probably something you shouldn't watch/read unless you don't have anything better to do.
2.5 Stars: Not necessarily better or worse than 4 or 3 but I just don't understand why I watched/read this. It probably is shit though.
2 Stars: Why does this crap exist?
1.5 Stars: Worse than shit.
1 Star: Warning: Could cause permanent damage to your view of anime.
0.5 Stars: I don't even...
0 Stars: Either I havn't thought up a rating, not finished it, not watched enough of it, or it's actually so bad it's not even worth rating 0.5.
*Warning: May Contain Spoilers*
*1: Naruto will always hold a special place for me as it was the second anime I watched (excluding pokemon) and got me into anime, hence the 4.5. Shippuden is a lot of good and horrible stuff piled together, I keep changing my mind whether to have it at 3.5-4.
*2: Bleach: As Naruto, Bleach has a special place as the first anime I watched. I loved Bleach, and I wish I could unwatch the newer stuff but I can't. After the last Grimmjow fight, everything plummeted to the ground. Even the fight with Ulquiorra I found crap. I pushed through for the sake of all the time I spent on Bleach but I wish I hadn't. Bleach would probably have had a 5 if I hadn't seen anything after the last Grimmjow fight. My hatred for the newer stuff probably has a lot to do with recent differing tastes from my old 14 year self as I was only 14 when I saw the first 80 episodes of Bleach. I doubt I could have enjoyed it as much as I did if I was 18 but I'm glad I saw it when I was 14.