Thoughts on the difference between Bush and Obama
The greatest benefit of a warrant is that it reduces capricious abuse of power. If an official knows he has to get a warrant (even a rubber-stamp warrant like you get from the FISA court) they might just hesitate a moment before deciding to bust down some doors just to teach someone a lesson.
Requiring warrants forces authorities to submit their requests to an outside authority. Even if they know that that outside authority is virtually guaranteed to approve it (again, see FISA), the mere fact they have to make the submission makes them stop for a moment to consider whether it is a good idea to go forward with it.
This was the problem I had with what Bush did and why I object to those who say Obama is perpetuating Bush's crimes. The biggest problem with Bush's model of governance is that he didn't believe he had to get the approval of ANYONE, even a rubber-stamp like the FISA court, to do what he wanted to do and he believed he didn't have to submit his actions for review by outside authority (congress or the courts).
Obama's proposed policies are completely contrary to that philosophy. He wants a clearly defined process for how to exercise the extraordinary powers of the presidency and he wants a well laid out plan for review and reversal. That is what makes him different from Bush and it is a HUGE difference.