Running around without a top will leave a couple of things exposed to the elements. Now since this is 4-Wheel & Off-Road you're reading and not some skin mag-we're talking about 4x4s here. So get your mind out of the gutter.
Removing the protection of a hardtop will let dust, dirt, mud, and water (or ice!) quickly rust and ruin your interior. Most electronic gadgets such as radios and speakers are especially prone to damage, not to mention that your expensive set of tools stored in a open truck can oxidize into oblivion.
A greater threat of being so open, though, is the five-finger discount. You know, having your personal stuff pilfered by the lowest of low-lifes. Since this country doesn't condone hand amputation as a theft deterrent, the only thing left is to lock up all valuables. Believe us, you'll need to carry a bit more weight in keys once you lose your truck's hardtop.
Here is a sampling of weather- and theft-resistant ideas to think about if you're going to start running around topless. And, by the way, don't forget the sun block.