There is then room for convergence: Buddhism thinks that grace is not necessary while Christianity thinks that grace is necessary, but they both agree that it is accomplished by our human effort.
I have been searching for a specific quote by St. Theophan that addresses this directly, but try as I might, I can't locate it. If my memory serves me, he says that we must strive with all of our power and intelligence and will to perfect ourselves, but also accept that without God, we can do nothing. The entire attitude of the Orthodox Christian should be therefore one of humility and surrender. As much as I (like you) have great respect for Buddhism, it does not much emphasize humility, and nowhere but in Orthodoxy have I seen such emphasis on humility. So I agree that there is some convergence, but I can't quite agree that both traditions agree that it is accomplished by human effort. Maybe we're not so far apart in our thinking, though, because a good deal of human effort is most definitely required of both!
Again, to quote St. Theophan:
"The Lord sees your needs and your efforts, and will give you a helping hand. He will support you and establish you as a soldier, fully armed and ready to go into battle. No support can be better than His. The greatest danger lies in the soul thinking that it can find this help within itself; then it will lose everything. Evil will dominate it again, eclipsing the light that as yet flickers but weakly in the soul, and it will extinguish the small flame which is still scarcely burning. The soul should realize how powerless it is alone; therefore, expecting nothing of itself, let it fall down in humility before God, and in its own heart recognize itself to be nothing. He who in humility puts himself in the hands of the merciful God, attracts the Lord to himself, and becomes strong in his strength. Although expecting everything from God and nothing from ourselves, we must nevertheless force ourselves to action, exerting all our strength, so as to create something to which the divine help may come, and which the divine power may encompass. Grace is already present within us, but it will only act after man has himself acted, filling his powerlessness with its own power."--pp. 136-7, "The Art of Prayer - An Orthodox Anthology"
I wonder though... why do Christians think that all meditational spiritual practice has to involve communication with God?
More St. Theophan...
"First of all it must be understood that it is the duty of all Christians - especially of those whose calling dedicates them to the spiritual life - to strive always and in every way to be united with God, their creator, lover, benefactor, and their supreme good, by whom and for whom they were created. This is because the centre and the final purpose of the soul, which God created, must be God Himself alone, and nothing else - God, from whom the soul has received its life and its nature, and for whom it must eternally live."--p.46, "The Art of Prayer - An Orthodox Anthology"
Mahayana Buddhists cultivate compassion (or "love") for other beings to the highest possible degree. I agree though that they are assuming "persons" even thought they don't admit it.
I don't give the Mahayana Buddhists enough credit, perhaps. The Bodhisattva is a Buddhist example of surrender, love (compassion), and self-sacrifice.