The StarCraft II World Championship Series is Blizzard's official method of conglomerating numerous tournaments, including their own, into a canonized ranking system. Players get points for winning various Intel Extreme Masters, Red Bull Battle Grounds, DreamHack events, GSL seasons, and so forth. Beyond the prize money of each event, points are awarded to sort a global standings list. These points, beyond bragging rights, lead to an invitation to the year's final tournament at BlizzCon.
The system has drawn some criticism, however. One specific complaint is that players are allowed to partake in any region of their choosing. This seems to lead to tactical placement of players relative to other ones, rather than actual geography. Moreover, this allows players to join in servers that they are not anywhere near to, introducing lag in the online components. If I remember correctly, the rules stated that, unless both players chose to play on a server that was outside the region (ex: a South Korean server for two competitors in WCS America), the server would default to the region (America in the previous example). For 2015, Blizzard is requiring that all players must be legal residents of the region they choose to play in. The reasons for this decision do not seem to be publicly explained, but it should discourage the shuffling of players for logistical advantages.
The other, major change is that all participants of WCS 2015 need to qualify. Previously, if I (again) remember correctly, while points were reset, some placements in leagues carried over. This time, if a player is in any given league, they fought to get there from the very bottom. If anything, I expect this became necessary when the decision was made to change residency requirements.
WCS 2014 isn't over yet, though. It will close with BlizzCon on November 8th.