Tattoos are very popular these days and it seems like everyone has a friend who draws that can design a tattoo. Just because you can draw doesn't mean you can draw a tattoo, there are aspects that are very specific to tattooing that the average artist doesn't understand.
A tattoo must be drawn to fit the space it is intended for. The body is made up of specific areas, long, curved, rounded, and each of them has a specific flow that connects it to the other body parts. Placing a design in an area where it doesn't fit can make it look awkward or forced for example a long straight design is best placed on an arm or leg not a curved pectoral muscle.
As well as being drawn to fit tattoos must have what is known as flow. The body has lines and contours that it must fit but within the design itself the pieces have to fit together like a puzzle. Lettering that is just thrown in can look like a poor cover up, too much going on can make it cluttered and confusing. Using flow can lead the eye into the piece and help to keep it there and focused on the message rather than trying to find what it is in the first place.
Tattoos can only be so small and so detailed. Each artist has their own skill level and some are more talented than others when it comes to detail, but at the end of the day they are creating in skin and not paper. As skin ages it changes texture and loses shape, this affects the tattoo the same way and the ink will spread and fade. Japanese tattooing is a great example where gaps are intentionally left to help designs last. Anything that is too small or over detailed may end up as a black blob within 5 years if it is too small and only an experienced tattoo artist will know the difference.
Tattoo design also needs to take into account the canvas that it's going on. Tattoo ink is transparent and if you try and put light colors on dark skin once the skin grows over it they will disappear. Therefore a white tattoo is only rarely successful on the palest of skins and if skin is extremely dark it is often hard to see anything other than vague shapes and texturing.
A tattoo design comes in three parts -the sketch, the line work and the finished piece. Ideally a sketch should show the construction of the idea, then the line work should act as a coloring book for the artist to work from, and finally a finished piece can help show color combinations and shading. The tattoo artist needs all of these if the design is complicated to be able to tattoo exactly what you want. Often artists will skip one or two of these, you may only see a line drawing from them, if they feel the design doesn't need explaining or if it is simple enough that they don't need to sketch it out first.