For this year, 2009, the difference is: There are over 22 MAJOR tax laws. This is not counting the dozens and dozens of laws which included the basic annual changes in tax codes.
This year, 2009, there are a number of complicated tax credits, that must be understood, completely, in order to successfully take the credits.
That is not to say that the major tax software companies won't include the new tax credits, because, they will, however, your ability to understand the more complicated tax laws is the concern.
What if the IRS didn't care if you make a mistake in THIER favor? However, if you make an error in YOUR favor, you will be receiving a letter in the mail, informing you of the changes!
Again, that is not to say that the IRS won't inform you, that you have a larger refund coming. But in order to do that, they (or the computer) will take a CLOSER look at your return, before a check is mailed.
If you find an online service which has a confidential policy, (in writting) and you can verify their professional status, it sure beats, waiting at the tax office to have your taxes done!