If the genie merely alters the present to conform to your wishes, you can easily run into unintended consequences.

The other problem is that divining someone's intent is tricky business. A person often has a dozen impulses at cross-purposes to one another and the interpretation of your wish will likely vary depending on how much sleep you got and what you had for lunch. There's a sci-fi short story Oddy and Id that examines a curious case of a man with luck so amazing that the universe bends to satisfy him. I won't spoil it, but I think it brings up a relevant point.