By extending and basically making permanent the Bush tax cuts in December, Obama took the issue off the table as an attack line in the 2012 election.
Now Mitt Romney or whomever the Republican nominee is can say "I believe that we should make the tax cuts permanent and so does my opponent because he extended them"
If anyone believes that the Bush tax cuts wont be made permanent and that it is still an issue that can be used against Republicans, they are dreaming. Do you think Obama would be willing to go into an election having allowed taxes to increase everybody? No way. The tax cuts will keep being extended.
What should have been done is to allow the tax cuts to expire for everybody back in December and then make Republicans vote on a bill that would have restored the tax cuts for the middle class. If they voted against it, they could have been blamed for a tax increase on the middle class.
Instead, Obama took the issue off the table and gave Republicans a free ride by caving on the issue.