Larges images (even if they are resized down: computer still have to load the image as if it was big)
Flash or Java applets
Videos (like YouTube)
Offensive language or flaming
But the most important thing is the max size is 550 x 150 pixels. Your total signature height (all images, text, and blank lines combined) should not be taller than that!
If it is we will PM you and remove the signature.
You may say my signature is too big but if you resize the window it downsizes.
