The most important voice has been cast for the upcoming Marvel's Guardians of the Galaxy animated series: Groot. Yes, the character who utters those three magical words will be be voiced by the classicaly trained Kevin Michael Richardson.
Richardson is no stranger to Groot, having voiced the character in the Ultimate Spider-Man, Avengers Assemble, and Hulk and the Agents of S.M.A.S.H. series, plus the Disney Infinity: Marvel Super Heroes videogame. You might recognize his voice from The Cleveland Show, (Cleveland Brown, Jr); or one of his many other voices including The Shredder (2012's Teenage Mutant Ninja Turtles), Bulkhead (Transformers Prime), Kilowog (Green Lantern: The Animated Series), Trigon (Teen Titans, Teen Titans Go!), etc.
Richardson is the first cast member announced for the upcoming series. However, hot on the heels of that announcement came news for Groot's partner-in-crime, Rocket Raccoon who will be played by Trevor Devall. Much of Devall's work has been associated with localized anime (eg. Mu La Flaga from Gundam SEED) and roles in Transformers Cybertron and Transfomers Energon.
** Zechs slams the bullpen door open and bursts into the room**
Zechs: sd you weren't going to end it there were you?
sd: Uh… well yeah, I…
Zechs: WHAT ABOUT HIS MOST IMPORTANT ROLES FOOL?!
sd: *blinks* Oh okay… Devall played Hoity Toity, Fancy Pants, Iron Will, Thunderlane, and an Instructional film announcer in My Little Pony: Friendship is Magic.
Zechs: Good! Don't make that mistake again!
** Zechs skips out of the bullpen **
He also played Rocket Raccoon in the Ultimate Spider-Man animated series (Rocket Raccoon is played by Seth Green in Hulk and the Agents of S.M.A.S.H.).
Marvel's Guardians of the Galaxy is slated to air on DisneyXD sometime in 2015.