I enjoy the challenges as its helping me to learn all the varied areas instead of just sticking to modelling. The variety is cool too as it pushes you to do other things - is this bad for most newbies who just want to make game models though?
Maybe as suggested splitting the money across first and second or first, second and third would help encourage people to try more. It can be a little offputting when you work your guts out and then the winner is not who you expected (not saying that is wrong, it is just a bit discouraging).
I dont have a problem with the timespan for the current challenges but if you were to simplify each challenge then maybe a shorter timespan could be acceptable.
As already mentioned visibility will always help