Congratulations on giving up the diet coke bida. I've given it up too (5 weeks now).
WastedThermos, you asked what is so bad about diet coke and other sodas. The main reason I gave up the diet coke is because I noticed some worrying things that were directly related to how much dc I drank, and my family noticed them too.
Basically, dc seems to increase my cravings. I have more cravings for sweets, chips and general junk foods when I drink dc regularly

. In fact since I have basically given it up, my cravings have become virtually non-existent.
Dc also seems to affect my mood and behaviour. When I drink a lot of it, I am more depressive

, irritible and aggressive

. I make myself more isolated as well. My mother works in mental health, and says that she has definately noticed that excessive consumption of real coke makes a definate impact on mood and behaviour (one of her clients drinks a 2litre bottle a day)
And finally, apparently the phosphoric acid in all cola drinks can adversely affect bone density in women. This could lead to a greater risk of fractures or osteoporosis.
The suggestions that the aspartame in diet drinks (and many other things) can cause cancer in humans is something that has been debated and studied a lot. The opinion of the majority is that cancer is not caused by aspartame from foods and drinks. However, it is an additive, and all additives have been shown to affect behaviour and mood, so I think it is best to avoid them when possible. By no means do I stop eating anything with additives, that is virtually impossible and certainly impractical, but I do try to reduce my consumption of soft drinks and other things with lots of artificial colourings, sweeteners or preservatives.