It does not affect safety.

[notlangley]

Transaero spokesman Konstantin Tyurkin said all that was missing from the wing was the cover to a hatch workers use to check the left engine. It was not immediately clear who had misplaced the hefty piece of equipment or why.
"This was a technical element. Its absence doesn't prevent takeoff," said Tyurkin. "It does not affect safety." (http://www.theage.com.au/travel/travel-incidents/panicked-passengers-abandon-plane-over-hole-in-engine-20120322-1vl9i.html)

[KBPsen]

The man is quite right, it does not affect safety. Up to 4 of these access doors may be missing on each pylon.

[Sunnyjohn]

The man is quite right, it does not affect safety. Up to 4 of these access doors may be missing on each pylon. . . . unless it happens to fall on you on its way down.