Not to spoil all the fun, but:
Cure for diseases: problem with that is that we are overpopulated now, who is going to determine who gets food and who doesn't? We need natural selection to insure the continuation of the species. Simply using every square inch of the earth's surface for people to stand on will spell the end of mankind.
Living Forever: Can you imagine meeting someone, marrying them, having kids, and watching them die, thousands of times over and over again?
I dated a girl a long time ago and a family friend was this lady. I met her one night and she told me she was 94, she didn't look a day over 50, I kid you not. She didn't have wrinkles, didn't have grey hair (didn't colour it either), didn't have osteoporosis, she actually looked younger than my gf's parents.
Know what she said to us? SHe's ready to die. She saw 2 husbands die. She saw all her kids die. She has even seen her grandkids die. All her lifelong friends are gone, and there are few people left that she considered friends and most of them were heading for the old age home.
11 months later she was dead. Heart failure but her physical 6 months previous didn't turn up anything wrong with her heart (and they checked).
What's that saying? Be careful what you wish for, you might just get it.....