Hoppers can go left, right and down. But there is one direction missing, UP!!! There should be a feature were hoppers could transfer items vertically. This would make it a lot easier to move items and dropper elevators would be obsolete.
This would be nice, but I can't see much point. Dropper towers work well enough, and as EnderCreeper says, they wouldn't make sense logically as they'd be upside down.
Another point I will make is that to push items up, it makes sense to have mechanical/electrical features like redstone.