There are lot of specializations in MBA and more keep coming every year. But, most of them can fit in the following broad categories:
6. Information Systems
I think the main consideration should be what your interest is. You have to answer, I am willing to go into say, marketing and stick to it as your career, lifestyle and enjoy it ?
For anyone who has non-Business school background, I would strongly recommend taking all the introductory courses for all the above 6 categories in the first 2 semesters of MBA. This will help to gauage your own strengths and weakness. Then, you can start your specilazations from the third semester onwards.
The hot areas (ofcourse more pay) are Management Consulting (sub speciality in Management), Finance and Marketing. The more the specialization, more salary it is. At the same time, in the economy like this, its very hard to find specialized jobs. A few of the hot focused specializations include -- Entrepreneurship, International management and Marketing research.