To my mind, the worst effect is this: you may well lower your basal metabolic rate (through mechanisms not entirely understood). A drop of 25% in BMR can occur with diet of as much as 1300 calories per day. Thus, if a normal person of your height, lean body mass, and activity level might burn off 2000 calories per day, you'd be burning 1500. This effect may never entirely be erased in some (all?) dieters. So with each Low-calorie diet yo-yo dieters go on, they could permanently lose, say, 10% of their BMR...going from burning 2000 to 1800 to 1620 to 1458 to 1320 calories over a course of four diets. Any more than that eaten, and they'd either gain weight or have to exercise a lot to make up for the discrepency. This has happened to me, so I can attest to it from that perspective, but I just did a lot of research on it in the medical literature, too--it's a very real effect, demonstrated repeatedly in men, women, and lab rats. If you're taking in 2000 calories a day or so, I wouldn't worry--probably that's enough to keep BMR from falling.
If you take in less protein than you need, you may start to lose lean body mass, a loss that may further reduce your metabolic rate (and could render you weak or prone to injury or illness)
The chances of ingesting on super-low calories sufficient protein to repair the heart and organs and muscles, sufficient fat to keep the brain healthy, and sufficient vitamins and minerals to keep all the other bodily functions going at peak efficiency are slim. There is some evidence a chronically lowered caloric intake lengthens life...and there is some evidence that it significantly shortens life. Wish I knew the answer to this mystery!
Whatever changes you make to your way of eating now, you'll probably have to keep up forever in order to keep off any weight you lose. So do try and eat as healthily as possible--eat veggies, get enough protein to support your LBM, avoid the high-carb foods most of the time, never eat them in isolation, get enough calories to fuel a healthy lifestyle, etc.
HTH
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