Your home needs a solid footing. If the land you're looking at has been filled in at any time in the recent past, it could be difficult to find solid, undisturbed ground to rest the footings of your home on. Without a solid footing, the ground will continue to settle after your home has been built, which will lead to critical structural failures and foundation cracking.
But not that solid! Bedrock can be found fairly close to ground surface in some areas. While not impossible to dig through, it will be more expensive, depending on just how deep your foundation needs to go. Ask nearby land owners or neighbors if they have had problems with stony soil or bedrock!
Clay is great for kids but not for construction. If the clay content of your soil is high, you run the risk of foundation cracks and a leaky basement. This is due in part to the expansive qualities of clay, which takes up less space when dry and expands when wet. Unstable soil = unstable home and unhappy homeowner.
Everything the light touches isn't necessarily your kingdom. Knowing the exact boundaries of a prospective property is important, so hiring a surveyer is a must. Your surveyer will not only mark off the exact boundaries, but he will also let you know of any potential problems he sees with the land.