As much as you'd imagine a company like Riot would at least make a little weed joke in its latest 4.20 update, there's not a one to be found in the lengthy patch notes that helps kick off the season 5 pre-season.
The big changes with this patch focus on items, the jungle and objectives on the Summoner's Rift. As well as it's new aesthetics, the game is being tweaked heavily to offer more strategic diversity to gamers, hopefully allowing more people to play with different styles and shake up the ever evolving meta-game.
For a quick run down, here's a spotlight video. If you want more information, the full patch notes can be found here.
Some of the most obvious changes are that spirit items are all gone. Dead, disappeared. Gone forever (probably). Smite gets buffed by their replacements however.
Monsters and neutral mobs are now tougher and there's a crab that hangs out in the river now. Smiting the different mobs will land you a little upgrade, making it more versatile for junglers to take the clearing spell.
Baron has also been improved, giving you a faster recall and the ability to boost your minions. Though he is tougher, so be careful.