As you've probably noticed by now, I rate everything I review on a 5-star scale. I've never bothered explaining what the ratings mean, though, so just for the record, I'm going to detail the various ratings, what they mean, and examples of shows that fall into each category. Hopefully that should make the scoring process a little clearer.
A five-star title belongs in everyone's collection - it's either suitably unique or otherwise impressive in story and presentation that anyone should be able to get full value out of watching it. They'll tend to be shows that make you think a little more - pure comedy in itself tends to have less broad appeal, and that stops it from being five-star material. Examples of five-star shows include Crest of the Stars and Paranoia Agent.
A four-star title is still hugely impressive, but contains elements that may ruin its appeal to fans of certain genres. They may be serious, they may be slapstick, but either way they'll be thoroughly enjoyable to fans of whatever genre the show is in, and have enough of interest about them that there's some scope for appealing to people who usually wouldn't appreciate "that sort of thing". Examples of four-star shows are Girls Bravo - essential if you enjoy fanservice-heavy or slapstick comedy - or Mushi-shi, where pacing and the unusual nature of the stories works against the series a little.
Probably about half of what I review ends up in the three-star category. Shows that land here are targeted fairly squarely at fans of particular genres, and stick to the usual formulas and stereotypes enough that they won't have much appeal outside of those target groups. That doesn't mean they're inherently bad - look at the comments on many three-star shows and you'll see I've enjoyed them (often despite myself) - but they're shows that won't appeal to everyone. If you've enjoyed similar shows in the past, though, you'll usually be safe enough picking up a three-star show. Examples include Basilisk or Elemental Gelade - both decent enough within their own genres and that will entertain fans of nack'n'slash or fantasy stories respectively, but won't hold much interest outside those groups.
Once upon a time, you would usually find two stars reserved for individual volumes or short OVAs - back in the days when I was exclusively covering physical releases, there was only one 13+ episode series that consistently scored this low. Now we're into the simulcast / digital era, though, there seem to be a lot more of them. A two-star release is seriously flawed - there's something about the presentation or storytelling that makes it a chore to watch, be it unappealing characters, storylines that make no sense or that throw "surprises" in with no explanation of how or why, or an insistence on taking far too long to get to the point. Two-star shows nearly always have a core of loyal fans, but outside that core the show will more often be met with derision than appreciation. Examples include Tenjho Tenge, with its interminable pointless fighting and flashbacks; parts of Naruto, where the series has been known to take 6-7 episodes to cover 30 minutes of "real" time; or Asobi ni Ikuyo!, which takes the established harem genre and does absolutely nothing of value with it.
The worst of the worst, usually one-shot OVAs that seem to have been released for no good reason. They'll have nonsensical storytelling, poor production values, and be poor enough to be unwatchable for a number of reasons. Fortunately, I don't see many of these, but examples include Psycho Diver and Zeoraima.
No stars. I've never seen anything this bad, on DVD or simulcast, and the Maker willing I never will, but I did find some stuff this woeful back when I was still covering fansubs. The likes of Bobobo-bo Bobobo or Eiken would fit the bill if I was having to score them today.