High Landing Weights:
Higher weights still mean a higher approach speed requirement and slower acceleration. A 10% increase in weight creates a 10% or so increase in landing distance, with a higher speed of approach, which in turn creates more momentum. A 10% increase in weight increases the stall speed by about 5%
For takeoffs where the liftoff speed is about 15% above the stalling speed, say we have a 10% increase in weight; this increase in weight would translate to roughly 20% increase in the take off run.
Keep in mind that as air density decreases both engine and aerodynamic performance decrease; (now, at optimum altitudes, engines do operate more efficiently as per performance limits of specific aircraft; an increase in air density means a lower density altitude) this is specifically applicable to take offs, where at higher altitudes it takes more runway to take off. Density altitude will also affect aircraft performance, and this includes on approach.
An uphill slope will increase the take off ground run and a downhill slope will increase landing ground run. The weight of the aircraft is therefore a very important factor to consider. Sure you can land at maximum landing weight, but it is not advisable to have more weight than you really need and have calculated for. Of course grass or snow can increase ground roll, due to less effective brakes, despite increased rolling resistance, so your weight is still a practical and critical thing to consider.