Re: colour me cynical
There's an astounding amount of FUD around the entire diet industry... if you were the type to see conspiracy everywhere you'd almost think that dieticians were more interested in keeping themselves in a career than any long term sense.
Fat in your diet: Too much, obviously bad but largely because you're likely to be eating too much unbalanced food and likely to not be doing enough exercise... and as rightly pointed out in another article on El Reg, very little of your body's energy is needed to maintain body warmth compared to even 50-75 years ago, e.g. within a generation. Certain types of fats have been proven to be very bad for you, e.g. trans-fats / hydrogenated fats and while these are great for the bulk food industry as they are very cheap and double as a preservative they are very bad for your body. Luckily there is growing awareness of this and while most governments haven't mandated clear, honest, labelling of these or banning them, this situation is improving. One very inportant point about fats in your diet is that it is far from the case that fat goes straight from your food to your own fat cells, there's a huge number of steps that the intaken fat goes through and the more steps the better as this requires more energy to process and the energy from the result tends to be eventually used more evenly throughout your day which helps to keep your hunger at bay as well. This is one of the reasons a good solid fry up keeps hunger at bay much longer than a bowl of cereal, a yoghurt and a fruit smoothie.
Refined sugars: These are best avoided as much as possible however in order to substitute for the lack of food flavour from removing fats in food the food industry has added sugar instead and this sugar is far worse for you than the fat it replaces. Refined sugar requires very little energy to process / break down and as a result produces an instant energy hit but no lasting, or spread over time, benefits. The less refined the sugar the better it is for you and the longer the period over which the sugars will be released. This is why natural fruit sugars or honey is far better for you than refined (cane or beet) sugar.
Calories: While it's useful to have some form of indicator as the nutritional value of food, there is a world of difference between a boiler and a digestive system. As a result a 100 calorie chocolate bar and a 100 calorie oat bar while having similar calorific values, produce vastly different results. The chocolate bar will be loaded with instant energy refined sugars which if you need an instant energy hit isn't a bad thing but very poor for a longer term snack and won't satisfy hunger in nearly the same way. The oat bar requires much more energy to break down and while the nominal calorific value is the same, your body will use much more energy to break it down and the results are spread over a much longer time. There is a noticeable lag between eating and your body's signals telling you that you've eaten enough, which is one of the reasons that eating slower works, similar to not eating while distracted (working, driving, watching tv) as we tend to not pay attention to our body's signals at this time.
Detox diets: These work solely due to the fact that you're cutting out the crap from your diet and eating less for a bit. There are no "super foods" or other rubbish, our bodies have had millions of years perfecting amazing detox mechanisms, eating freshly-marinated-quid-puree (or whatever today's celebrity inflicted detox diet is), doesn't help any. The other risk of a heavily calorie restricted diet is that your body goes into "starvation mode" where it hangs onto food for longer to wring out as much as it can for it (constipation often results) and then has a digestive fun day when you stop the starvation and supply enough food where it will continue to wring out more of the food... which is why the weight piles back on and the cause of yoyo dieting. If you must starve yourself, do it for a couple of days time at the most and then eat normally for the rest of the week.
Typed up while eating a nice sugary snack because it's oh so tasty... mmm... dammit.