Cages can be hot from a number of angles; degradation, captivity, kenneling for pup play, etc, but they've never really done much for me in and of themselves. I think mixing cages and watersports or some other activity that the sub has no choice but to endure due to the confined space could be wicked hot, but the bondage pig in me wants more restrictive storage.
That being said, the bondage pig in me LOVES that there are dozens of tie-points and potential positions depending on the dimensions of a cage. Here are a few that I can't even glance at without getting rock-hard. One of the hotter concepts here is having part of your body outside the cage, almost tauntingly.