Riot are using the period in-between Season 3 and Season 4 to get players used to some substantial game changes.
Preseason, the time in-between League of Legends competitive "seasons" of play, is the best time for developer Riot to roll out substantial changes to the game. This time around, Riot has had a long, hard look at the support and jungle roles, the general game flow, and some of the pregame choices. Check out the video to the right for a nice little summary.
Basically, playing Support and Jungle late game is not very fun, because of how easily they fall behind their teammates in gold and experience. For support, Riot is addressing these concerns by heavily adjusting support gold to put more money in the pockets of duo-lane supports. Along with this, it is making changes to help core support champions scale their utility instead of their damage, and reinforce their unique identity while opening up new items that put all the new gold to good use and make their impact felt in late-game teamfights. Check out the new support items here.
For junglers, Riot is adding new masteries, as well as adjusting overall experience flow and changing up previous incarnations of jungler-specific items, such as giving these items a unique passive that increases the ammount of gold jungle creeps give. Check out the jungle changes here.
Changes to the game's flow include some physical changes to the actual Summoner's Rift map (such as the removal of certain areas of bush), changes to how wards work, and the addition of the "trinket" item. A trinket is a free item that "evolves" at level 9, and can be upgraded a second time for an additional gold fee. Check out the game flow changes here
Riot have put together a full preseason website that details all of the upcoming changes, which you can check out here.
Source: Riot Games