Since the personal exemptions are zero on the 1040, there are no allowances on the W4. The withholding tables broke from past practices of using a number much smaller than the standard deduction so workers were more likely to have a cushion of extra withholdings. The new tables use the full 12,400 standard deduction, so that cushion is gone.
A single worker with one job who just checks single on the new W4 then signs it, will be withheld very close to what their actual tax will be. Tell your clients that, and if they want a cushion they'll have to enter an amount on the extra withholding line to ensure a refund.
The new form should also prevent the perennial disaster we've all seen, where a couple gets married and both changed their W4s to married, and end up owing a lot when they file their first joint return. A lot of wedding sites/magazines were actually recommending the newlyweds do this.
And it's designed to help the person who works several jobs, and has no withholding on the lower earning jobs. Now they can have their main job do all of the withholding for all of their jobs.
To me the biggest downfall is going to be kids working summer jobs having unnecessary withholding then having to file a return to get it refunded. They should have put a 4th checkbox at the top, that says, I am single and expect my total income to be less than 12,400. This would let employers know to not withhold any taxes. If you read the instructions, your supposed to write 'EXEMPT' in a blank space under 4(c), which is really poor form design.