Jagged lines

if you go to the settings screen, click display then hardware. tick the box that says anti-alias. basically it means smoothing by blending edges. it can be a bit system heavy - but worth it, so just turn down something else.

if it's in the aircraft textures it could be two things a) the textures are crap, or b) you're unsure what a mip map is.

a mip map is pure evil. some textures have them,thankfully most don't. all the default aircraft have mipmaps, you can tell because when you move away they suddenly become blurry and jaggy.