Good question. There are some downsides, though I think they're outweighed by the upsides:
1. The tax-benefit is in the future, thus uncertain. At one point, it was thought that Social Security payments would never be taxable; they now are. It is possible that Congress may at some point decide to tax Roth IRA distributions. That said, there'd be a lot of opposition, and there would probably be a Constitutional challenge.
2. As I noted in
this thread, if you are close to a breakpoint for obtaining the retirement savers credit, placing money in a Roth vs. a Traditional IRA would not reduce your AGI, thus not help you out in meeting the breakpoint. If you're within a couple hundred dollars of a breakpoint, it may be worth it to divert that money to a Traditional IRA (that said, many companies have large minimums for establishing Traditional IRAs, relative to what would be optimum for you).
3. If you expect your income to decline from the date or Roth IRA contributions, or taxes to decline, it may be more wise to contribute to a Traditional IRA.
4. If your employer matches your contributions, you should contribute to a 401(k) up to the point of matching,
then, with your remaining disposable income, contribute to the Roth IRA up to the limit, then with whatever's left over (if anything) contribute to the 401(k). That is: 401(k) to max out matches => Roth IRA up to max => 401(k) as much as possible.
All that said, I still think a Roth IRA is the best way to go (with the one exception for 2, and that would mean contributing as little to a Traditional as you could). With a Roth, the longer you leave your money in, the greater the benefit becomes, because of tax-free (not tax-deferred) growth. Since you're 24, it is highly unlikely that your income in retirement will be lower than it now is. Furthermore, it would take a monumental leap of Faith to believe that somehow taxes aren't going to be getting higher and higher.
Other benefits of a Roth IRA: (1) there are no minimum required distributions during your lifetime, so you don't have to start taking money out at 70.5 if you don't want to; (2) You can always take out your
basis in a Roth IRA without any taxes or penalty.