Offering critique (advice) to sig makers
This is a revised/shortened version of link64's critique guide here, I suggest you all read it if you want to become good at giving advice - http://www.friendcodes.com/forums/member-created-art/69208-critique-guide.html
It's terrible when good work is ignored for something that isn't actually that good. It might be a sig that has a render of something popular so people will instantly say its good without really looking at the sig itself.
These are the things that actually make good sigs:
-Depth
-Flow
-Concept
-Effects
-Text/Misc
Going over these now, you will learn how to spot these attributes. This will also help you give good, constructive advice on someone's sig..
Depth: In a sig you should always have a focal point. This is what the viewer will be focused on. The main focal point in sigs usually is the render in the sig but not always. Good signature's focal points stand out the background and the effects. So in other words, things that are suppose to look further away in you sig should be blurrier make the sig look "deeper". This effect is called depth.
A sig displaying some good depth
Flow: Flow is a little harder to explain but it pretty much means that the render/effects in you sig seem to all move in a certain direction (flow in a certain direction). The direction of flow would depend on the render you are using. If you have a sig that is facing a certain direction (left or right) I would suggest making it flow in the opposite direction of that sig. In some case it will work the other way around.
A sig with 'flow'. Notice how the effects move in a certain direction.
Flow doesn't have to be linear (i.e. move in only one direction), it can burst from a point of focus and move outwards, or it can move it two directions away from the render. There are many types of flow.

Concept: The idea behind the sig.
This sig's concept is death.
As you can see the guy who made this sig pulled off the idea really well. Looks like he has just collapsed onto a table has blood running from his mouth blood then running across the glass table. You should really take you time when looking at peoples concepts. Have a look at the render/stocks etc that they used and see how much effort really went into the sig.
Effects: Effects really do make a great sig. There are so many effects a sig maker can use. A lot of people tend to use Effect C4Ds now but effects can also be added by smudging and clipping masks. Brushes rarely look good unless they are for lighting or in a vector (block color) sig.
Sexy effects =O
Text/Misc: Text is in aid to everything. It gives concept by explaining it, effects via the look of the text and even flow when placed in the right spot. When doing text try to make the colours match the sig, blend it in and keep the fonts simple and the text to a reasonable size. Good things to look for in text: Clipping masks, Gradient maps and Strokes. Borders: Some sigs look good with borders. Lighting: Good lighting is visually pleasing and gives a good focal.
Sorry to link64 for mutilating his guide, I wanted to sort of make it a non-personal explanation lol (but also I didn't rip this guide either, he asked me to use it so just keep that in mind before you attack me =P).