I am pleased to see that many good reasons for avoiding 401k to IRA rollovers have been mentioned by others in the group, so it looks like the word is getting out.
One reason, that hasn't been mentioned yet, to leave the funds in the 401k, rather than move to IRA, would be for simplification of tax return preparation down the road and increased confidence that you will not overpay taxes every year down the road.
For funds in a 401k, the plan custodian is nearly always responsible for correctly determining and reporting the taxable amount of any distribution. (ie the "taxable amount not determined" (box 2b) on 1099-R is unchecked and the taxable amount is correctly reported (in box 2a).
The 401k plan providers are required to track any non-taxable basis in the account, over the account's entire lifetime , and correctly apply the "general rule" to determine the taxable amount each year there is a distribution.
IRAs, on the other hand, will nearly always have box 2b checked. "Taxable amount not determined". Even if they report a taxable amount in box 2a, it should not be relied upon. The burden is on the taxpayer, not the IRA provider, to correctly determine the taxable amount -- which in general can be less than the gross distribution.
The issue is that far too many people will just type the amount of box 2a into their tax software and pay tax on the entire amount and not know the difference. This is even more common when surviving spouses take over responsibility for filing taxes after the passing of the first individual in a married couple.
-gauss