Yes, some privileges will come with the higher ranks and xp's e.g. posting news on the member's section, etc.
One idea is to use different shades of colour for the listing of members online on the right panel based on ranks.
As for the challenges, the next 3 are being coded at this very moment and should be out in a day or two.
Another 11 are in the pipeline. That adds up to a total of 28 (14 + 3 + 11) challenges.
We welcome ideas for challenges 29 onwards :)