All the other Surahs (correct me if i am wrong) starts in the name of Allah … i.e … 
… why is this Surah different from the rest? Why doesn’t it starts with … ?
This is the only Surah of the Quran to which Bismillah is not prefixed. Though the commentators have given different reasons for this, the correct one that which has been given by Imam Razi:rehm:: namely, this is because the Holy Prophet himself did not dictate it at the beginning of the Surah. Therefore the Companions did not prefix it and their successors followed them. This is a further proof of the fact that utmost care has been taken to keep the Quran intact so that it should remain in its complete and original form.
and then there is a saying of Hadhrat Ali 
that the tone of this surah does not demand Rehma of ALLAH which we see in the Bismillah 
Wallah-O-Alam!
dr. zakir naik's explanation is an interesting one....
he says that in this surah there is warning to non-believers and to muslims who were lazy....
the mood is aggressive and when one is angry he does not start with bismillah....
e.g. a muslim shud start all his work with bismillah but if a thief snatches a bag from the hand of a muslim and the muslim catches him, he wont start with bismillah but with straight words "give me back my bag"....
Thanks 