1. Paying interests too soon. If you file early, you will pay for your tax interests too soon. It is like giving to IRS your money sooner than you ought to. If you could keep your money until April 15 in an interest-bearing bank account, then you earn a few extra bucks for yourself.
2. Not being able to amend your return. Mistakes in filing tax reports are commonplace. So if you file early and later on discover some errors, you have to mail again for the second time. This would take some time. Plus, more likely you have already used up some of the tax refunds you received for filing early. Do you have enough money to pay a sudden tax liability?
3. Not being able to report tax using a revised IRS form. Most tax legislation happen within the year and changes in the IRS forms may not be reflected in the forms you use if you file early. This would leave you paying attention for any tax legislation update after you make an early filing so you can make the necessary adjustments if you need to.
Filing your tax returns early may not work for you if your taxes are complicated. You will give your self a great favor if you wait for April 15th.