Do not place heavy items such as baker's racks or chairs there, especially if you have children. Other things that are best left off your island are microwaves and toaster ovens because they do tend to take up a lot of space – so adding them will make the kitchen feel cluttered.
Another thing that should be avoided is extra appliances, especially those that don't get used often (although this one is totally up for debate). And last but certainly not least – no food. At least keep anything edible away from the edge where unwanted attention might be attracted.