There are some super thin light panels around, but when it comes down to how good they are, application is everything. Generally speaking there are two different types of light box:
Tube lit light boxes
Fluorescent tubes provide the light. You can get both back lit and side / edge lit light boxes. The main advantage of this type of light box is that whiteness of the light can be regulated by the specification of tube you fit into them. Typically 5000k is classed as colour corrected which means that if you are trying to reproduce specific colour shades / tones, the light from the tubes will not distort what you can measure.
Tube lit also produce a greater intensity of light (not enough to give you a headache or anything) which penetrates through paper up to 450gsm. It makes them the only sensible choice for designers or artist who are looking for a tracing light box and really scrutinise the details in their work.
LED lit light boxes
If you are looking to display artwork on an illuminated light panel then LED will be much more suitable. Designed for high efficiency and constant use they are a great way to catch the eye of a potential customer. However, without the light intensity of the tube lit light boxes, your artwork needs producing on a clear film / acetate.
There are LED light panels on the market which have been designed for simple tracing work, but if you don't have the right tool for the job it can leave you wanting.
Although there are slim light boxes available for design work, if you go too thin you end up with something which can only really be used for display purposes. Designers really benefit from the colour corrected light; something which you don't get from LED light panels so be wary when choosing a light box - make sure it's fit for purpose.