It will just take longer to bake, won't be as pretty or interesting, and will have less crust. A tube pan or Bundt pan has much more surface area than a loaf pan, so the batter comes in more contact with the pan, and receives heat more directly. In a loaf pan, the batter comes in contact with the pan less and is poured more thickly in the pan, so it takes longer for the heat to penetrate to the center of the batter.

In fact, a tube or Bundt pan is often called for to help cook cakes faster and more evenly. With some of particularly dense recipes (banana bread, for example), you might not be able to get the interior cooked through before the outer areas are overcooked. Also, angel food cake needs the center wall to help support the structure of the cake throughout baking and cooling.

Because there is less surface contact, there will be less crust. In some cakes you might prefer less crust, but tube pans are sometimes specified to produce more crust and for the visual interest they create.

How much extra time it will take to cook depends on the dimensions of the pan you use. But a toothpick or skewer jabbed into the middle of the cake that comes out clean will tell you when it's done, no matter what the shape of the pan.