Well, I know that diet drinks bloat your tummy out. Most diet drinks contain phosphoric acid which rots the enamel from your teeth (read "FastFood Nation" by Eric Schlosser for more info). The phosphoric acid would be pretty hard on your system.
The caffeine in diet coke would make me hyper and I became addicted to diet coke.
All the diet drinks have sweeteners in them. Technically, they have less calories in them than regular sugar but you're actually better off with regular sugar than sweeteners. Your body recognises sugar and eventually will feel full but it won't recognise the sweeteners so you crave more of it and will probably drink more of it. That's why I don't believe in low fat food products.
I cut out all diet drinks, now drink three litres of water, all my skin problems have cleared up which no cream/lotion/potion was able to do! I tried Clinique, Neutrogenia etc etc but they didn't really do anything for my breakouts. I didn't have really bad acne but I could never get it totally clear. Now, I use warm face cloth and warm water as cleanser, cold water as my toner, vitamin E cream as my moisturiser morning and night and thats it! Natural is def best.