Well,
#1 it is against the tax regulations to file without a W-2 unless you check the box that says "I did not get a W-2" and include a "substitute W-2 form." This will trigger an investigation of your company for why they didn't issue their employees a W-2--when in fact, they have until Jan 31 and it is you who are in the wrong.
#2 because of different tax treatments of different pre- and post-tax deductions, your box 1, box 3 and box 5 amounts are not always easy to predict from your pay stub. If you get it wrong, you have to file an amended return. Amended returns take 20 weeks to process, partly because there is a big backlog due to people filing with incorrect information.
#3, the IRS attempts to match your claimed income with the copy of your W-2 that the employer sends to the IRS. If the data does not match, you could get a letter from the IRS, or your refund could be delayed for additional verification.
#4, the IRS does not process any tax returns until January 23, even though turbotax will take it now. Since W-2s must be mailed by January 31, waiting will only actually cost you 8 or 9 days, not 25 days as it might appear.
#5, if you are claiming earned income credit or the additional child tax credit, there is a new law that requires the IRS to hold all such refunds until after Feb 15, so early filing won't get you ahead at all.
But certainly the software can't tell whether or not you have a real W-2.
