Dominion will be the biggest change to LoL since beta and will include a brand new format of gameplay. A new map has been made and the goal consists of capturing 5 points on the map and trying to hold on to them for as long as possible. Every team has resources that slowly decrease. The more points captured, the slower the rate of decrease. The first team to zero loses.
In addition, champions will start at level 3 and with more gold than before (around 1350 possibly more). This means that players can have a greater range of starting builds and implement a more diverse opening phase. Clearly, laning will be a thing of the past as I suspect players will go for ganks very early on.
Dominion essentially makes the game shorter and much more aggressive. It will be interesting to see if all champions are viable in this format as some champions are known for late game domination or are heavily dependent on gold for top tier items. Regardless, it is good to see Riot mix things up. HoN just went free to play a couple weeks ago. All of this is probably done to try to gain a greater following before DOTA2 comes out this Q4 (where it is expected to kill LoL and HoN).
The Dominion update is expected to arrive this September.