Southwest Airlines is apparently going to vary the price of Early Bird check-in by route instead of charging a flat $15. Some routes will go up to $25 starting August 29th, and this will be based on two things: the length of the route (so how much getting a more desirable seat is likely to matter to you) and demand for Early Bird Check-in on the route (how many people are already buying it).
This isn’t as big a deal as it seems, especially if you know how to play Southwest Airlines boarding strategically.