1. The tax status of your old 401(k) plan
Most traditional 401(k) plans operate on a tax-deferred basis: You get a tax deduction for your contributions to the plan in the year that you make them, and the money you have invested in the plan grows untaxed until you withdraw it.
If you have a tax-deferred plan, you’ll need to be sure to roll over your 401(k) to another tax-deferred account. Failure to do this can have really unpleasant tax consequences.
Say you were to roll a 401(k) into a Traditional IRA. This, generally, is completely acceptable as both of those types of accounts share the same type of tax-advantaged status. However, if you were to roll your pre-tax 401(k) into a Roth IRA, that’d be treated as a Roth conversion, and unpleasant consequences could ensue. Unless you made that move as the product of careful financial and tax planning, you’d be liable for a potentially significant tax bill.
2. The details of your new plan
If your new employer has a 401(k) plan as well, you should look to see if the new plan allows “roll-ins” from other qualified retirement plans. If it does, you’ll have the ability to seamlessly merge your previous 401(k) with your new one — assuming, again, that the tax status of the two accounts is the same.