Rolling resistance mainly. Grass tends to slow you down more quickly but also holds you back a bit on take off. You should factor into your performance numbers the fact that on grass you will need longer to get airbourne. Good smooth flat dry short grass will probably make very little difference; long wet bumpy grass can make loads of difference. I recall there are some official factors figures for these surfaces, but cannot remember where to find them; anyone know?
Also braking on wet grass can mean the wheels lock and you just skid along gracefully. It is difficult to remember cadence braking when you are skidding off the end of a runway.
Grass tends to be a bit more forgiving in cross wind not quite perfect landing I find.
Otherwise its the same as any other surface.
Sywell is a bit bumpy so beware of prop tip clearance and tailwheel fatigue, but other than that you should be able to get any GA a/c in an out without much trouble. If it is muddy the mud can get into the wheel bays of retractables and clog up he micro switches giving spurious gear warnings.
It can be good fun watching people who usually operate on 4,000metre tarmac landing on the shorter runways, down hill; still there is a fence to arrest the travel
