There is substantial differences between OCW and EdX. EdX, in addition of providing the educational contents for free online, will implement a new system of qualification. Not based on academic degrees but on specific courses certification. The way courses are accessed online will also be different with interactive quizzes and integrated materials. So, the whole delivery platform will be different.
from the MIT web site, regarding MITx, the predecessor to EdX
This seems to indicate that in future versions, the actual certification, which could be verifiable and possibly peformed ina controlled testing center would add signficant value to the course.
Another main benefit not available with OCW, which became clear during the first course offered by MIT to over 60,000 students, is collaborative learning. Students can work interactively to solve problem sets and discuss the material. They may also be able to interact with faculty.