"she saw a person, but did not know who he was, by reason of the form of his appearance, the difference of his clothes, and not expecting to see him alive; or through modesty, she might not look wistfully at him; and besides, her eyes were filled with tears, and swollen with weeping; so that she could not see clearly; and her eyes might be holden also, as the disciples were, that as yet she might not know him: so sometimes, in a spiritual sense, Christ is with, and near his people, and they know it not: Christ, as God, is omnipresent; he is every where, and in all places; the spiritual presence of Christ, is more or less, in some way or another, always in all his churches, and among his dear people; but the sight of him is not always alike to them, nor does he appear to them always in the same form; sometimes against them, at least in their apprehensions, nor always in a manner agreeably to their expectations; nor is his grace always discovered in the same way, nor has it the same effect" (John Gill's Exposition of the Bible).
"Knew not that it was Jesus. She was not expecting to see him. It was yet also twilight, and she could not see distinctly" (Barnes' Notes on the New Testament).
"And seeth Jesus standing It may be asked, Whence arose this mistake, that Mary does not recognize Jesus, with whom she must have been intimately acquainted? Some think that he appeared in a different form, but I think that the fault lay rather in the eyes of the women, as Luke (Luke 24:16) says of the two disciples, their eyes were withheld from knowing him We will not say, therefore, that Christ was continually assuming new shapes, like Proteus but that it is in the power of God, who gave eyes to men, to lessen their sharpness of vision whenever he thinks proper, that seeing they may not see" (Commentary on John-Vol. 2).
"Chrysostom suggests that she ‘turned herself’ because she saw in the angels’ looks that they saw Christ suddenly appearing behind her; but the preceding explanation seems better. Her not knowing Jesus might be accounted for by her absorbing grief. One who looked at white-robed angels, and saw nothing extraordinary, would give but a careless glance at the approaching figure, and might well fail to recognise Him. But probably, as in the case of the two travellers to Emmaus, her ‘eyes were holden,’ and the cause of non-recognition was not so much a change in Jesus as an operation on her" (Alexander MacLaren, Expositions of Holy Scripture: St John Chs. XV to XXI).
"Neither are we told why Mary was unable to recognize Jesus. There appears to have been something different about the resurrected Jesus which resulted in him not being immediately recognized even by those who had known him well. Something similar happens in John 21:4, as well as Luke 24:13-35, Luke 24:36-38, and Matt 28:17" (W. Hall Harris III, Exegetical Commentary on John 20). See http://bible.org/seriespage/exegetical-commentary-john-20
I agree with the position espoused by W. Hall Harris III.