The Oak Bluffs Town Beach is a pretty, small and calm beach which is very convenient if you're staying in or near OB, or visiting for the day. The beach itself is relatively narrow so it fills up quite a bit on nice days, with a mostly family-type crowd. But there's enough space so you don't feel crowded or squished.
The views are beautiful, swinging around to the OB harbor which houses hundreds of boats. Every now and then one of the big ferries or a cruise ship slowly floats by in the near distance. Aside from the clean beach and nice scenery, I'd say convenience is the main draw here, with the town so close by and a well-stocked packy (package store, aka liquor/convenience store) even closer.
Check out this web site for more information on the beaches of Martha's Vineyard.