Rome has defined the assumption of Mary as a dogma.
I've heard it said that Orthodox don't really think it ought to be declared a dogma. That it's not the sort of thing that ought to be binding.
On the other hand I've heard it said that Tradition in Orthodoxy is every bit as binding to the Orthodox as official dogma would be to Roman Catholics.
Along a similar line of thought, how ought I as Orthodox regard St George and the dragon? Am I to believe in dragons?
If it's not clearly defined by the Oecumenical Synods it's not the kind of dogmatic tradition that is 'binding'. I personally don't think that this is the kind of thing that really needs to be dogmatized, traditionally we have only dogmatized theologies directly relating to the nature of the Trinity or one of the Members of the Trinity, to dogmatize something like this would just be to create unnecessary division. Though I have no problems with this particular doctrine, to dogmatize it would be, at the very least, imprudent.