http://www.surrenderat20.net/p/current-pbe-balance-changes.html
The first towers are getting more health and a new armor mechanic. Basically, towers are harder to take down in the early game, especially by multiple people, but there is a gold reward for staying in lane and attacking the tower rather than roaming for kills. Turrets also deal considerably more damage, making tower dives much more risky.
The goal is to make the laning phase longer and keep people in lane with their opponent to allow for more interaction. Even if a midlane assassin gets three kills early on, they won't be able to take the tower because of all the extra health and armor it has, and they won't be able to roam for value because their lane opponent can shove back and get a bunch of gold while they're gone.
There's also tweaks to minion scaling to make it easier to win without Baron, and a reworked bounty system to make comebacks easier.