A Roth 401(k) [wikipedia.org] is to a traditional 401(k) as a Roth IRA is to a traditional IRA.
Roth 401(k): Post-tax contributions; money grows tax free; withdrawals are not taxed; employers are legally allowed to do the same things as with a traditional 401(k); same contribution limits as a traditional 401(k) [technically the contribution limit applies to the sum of all 401(k) contributions, traditional and Roth alike].
Excellent information; thanks! I had no idea such a thing existed. I wonder how many employers offer Roth 401(k) plans. I don't think mine does; then again, 10 minutes ago, I didn't know that Roth 401(k) plans even existed. Thanks for educating me. I should have looked it up before assuming it didn't exist; I'm sorry about that.
tl;dr Roth retirement accounts let you pay a 0% tax rate on all capital gains as long as you pay tax on "principal contributed" instead of "principal withdrawn" (because you're paying tax on the principal either way), unlike traditional retirement accounts, which make you pay tax on all gains. Why would the average person go traditional?
Indeed. This merits more investigation. Thanks!