the first most important effect of a infinite flat plane is that gravity would be finite however dose not become weaker as you go up because of this all light will form a parabola and eventually hit the ground at higher angles the light travels further it would allow you to see infinitely far by looking up there would be no horizon or sky instead strait up would let you see infinitely far away and because the light from the far away points bunches us it would have the same brightness as nearby points