If you are wanting to keep the low profile look, then very little. However, if you ever plan to take your LR3 off road, then those low profile tires are almost all but useless. Once you start shoping for good all terrain tires you find out that the selection for 19" wheels in that category is minimal at best. Coming down to 18"greatly increases the choices, gives you a more comfortable ride, and more side wall on your tire, making it a much more off road worthy tire.

The only limitation to an LR3's awesome off road performance right out of the showroom floor is the useless Good Year H/P factory tires. Especially if they are 19's.

 

There is greater tire selection going up or down one size (As Jantiz said for off road or All-terrain go down) and the prices are much less (up to $100/tire) for a better quality tire depending on what you select.