I have just been looking at the proposed revisions made in the definitions and diagnoses of mental illness. There are only two differences between mania and hypomania in the DSM-V and it is slightly different from the DSM-IV-R

Mania:

C. The mood disturbance is sufficiently severe to cause marked impairment in occupational functioning or in usual social activities or relationships with others, or to necessitate hospitalization to prevent harm to self or others, or there are psychotic features.

Hypomania:


D. The disturbance in mood and the change in functioning are observable by others or represent a subjective change.
E. The episode is not severe enough to cause marked impairment in social or occupational functioning, or to necessitate hospitalization, and there are no psychotic features.


In the past, the major difference between hypomania and mania was that for it to be mania it had to be severe enough that it required hospitalization.