I was told by an anonymous pigeon that I rate too lowly on some WB skins. With that in mind, I've tried to rate a skin more on its inherent merits rather than only whether I like it or not. So I've created a system to rate a skin. There are certain things I believe a skin must have to be useful, and lacking them means it is less than perfect.
1: Needs to be original. Recolored versions of Luna, Longhorn, etc are not original. If it's a MAC clone skin, it needs something that sets it apart from the crowd of other MAC clone skins (this includes Aqua, Tiger, et alius).
2: Needs to be complete. Menus and dialogues that look unfinished, or use ordinary windows objects show me that the skinner didn't care to take the time to finish his skin. A skin should cover all the bases, and be a complete makover. Missing parts are not acceptable.
3: Needs to be relatively high contrast. This may be personal opinion, or maybe not. Fact is, some very beautiful skins are rubbish as far as actual usability is concerned, because once you get them on the screen, you can't read certain items such as: Titlebars, Menu Items, Buttons and Tabs. This is especially true for the darker coloured skins. Dark should not mean hard to read.
4: Window controls should always be in the preview screenshot, as should the start menu. These are the most important things (even though personally I never see the start menu, as I use OD+). If these are not in the preview, then it is difficult to give a fair analysis without actually downloading and installing the skin, which is almost a form of blackmail on the author's part to get more downloads. Therefore, if you don't include these things in your screenshot, I have no choice but to rate lowly.
5: If the skin requires a specific wallpaper to look good, that wallpaper should be included with the skin if at all possible (ie, you own the wallpaper or have permission to distribute it). Having to make a seperate download artificially inflates the author's download count.
6: Fonts should be clearly readable and fit within the areas given by the skin. Some fonts are too large, and run off the sides of buttons, etc. This should be avoided. Fonts that overlap parts of the window controls (including titlebar text that strays outside its box) are a no-no.
7: If you have custom "controls", they need to not have too high a premium of screenspace. Keep them small. Label them clearly enough so they can be found and clicked on. I once used a skin for a day before I realized it had winamp controls on it. Additionally, offer a substyle without the controls, or offer some configuration. Not everyone uses the same software you do. Having a beautiful skin loaded down with controls for a program you don't use is rather pointless.
8: Keep down the clutter. I'm not saying all skins need to be simple, but too much noise can reduce the overall effect. Find a theme, and stick with it.
9: If I see "magic pink" showing anywhere, you lose several points right there.
10: This one is subjective. The graphics used for the various objects in the skin need to be cleanly drawn or rendered. Unexpected breaks, things that don't line up, fuzziness where fuzziness isn't expected, buttons that don't "sit naturally" on their background, all these things are unattractive and can ruin an otherwise beautiful skin.
So, mostly these are the things I'm looking at on skins. If a skin does well on all categories, it'll get a 9 or a 10. If it breaks the rules of all the categories, I'm not afraid to give it a 1.