I was playing MGS1 over again and have noticed a few repeat statements that have been said in the trailers for MGS4. Like when snake is held at gunpoint by Meryl and he says, "You still have the safety on rookie." And when Gray Fox says, "I'm neither enemy nor friend." like Drebin. What are your views on this.

Gray Fox and Drebin are not the only ones to use that line, Olga, Ames and (young) Ocelot all used it, theres something fishy about that quote.

And the rookie quote was just a joke from Kojima.

The "You still have the safety on rookie" is Snake's line and he used it in MGS1 when he faced Meryl from the first time. I believe Kojima uses it as a 'reminder' from the first game to point out that Snake's beliefs and habits haven't changed as well as his spoking.
The "I'm neither enemy nor friend" line was used by numerous characters over the game series as Roy Campbell said and there were some assumptions here on the forum that this has something to do with The Patriots. Like a motto that the members use.

another statement that's from mgs is said by liquid in the trailer with him and snake in the metal gears. he says "it's not over, not". the emphasis on "not yet obviously refers to when he said it in mgs while chasing snake in the jeep at the end of the game.

There's nothing really special about these repeated lines - they are merely throwbacks to the other games.

Kojima does this kind of thing all the time, and whilst it is a pretty cool feature ...they are not significant.

Maybe for a sense of continuity in the series?