They brought back several of the same voice actors who worked on G1 to reprise their characters. Most notably Corey Burton as Shockwave and Spike Witwicky; but also John Moshitta as Blurr, Susan Blu as Arcee and even Judd freaking Nelson for Rodimus Prime's few lines in TransWarped.
More Japanese voice cast, a few of whom are actually TF veterans, such as Roy Mustang as Ratchet and Urataros as Prowl to name two. Renji of Bleach as Lugnut and Fumihiko Tachiki as Prometheus Black/Meltdown as non-TF vets.
Screwed by the Network: Sure, three seasons and 42 episodes doesn't sound too bad... until you realize that the show was on the air for less than a year and a half.
What Could Have Been - Hot Shot and Red Alert were meant to be part of the main cast, but were replaced by Bumblebee and Ratchet. Rodimus was first choice for the 'jerkass on the Elite Guard' character, but this was apparently nixed by Hasbro.
Also, Hot Shot was going to recieve a toy later in the toyline, along with a triple changing Megatron, but both only made it to the prototype stage before the series was nixed for Revenge of the Fallen.
In the episode "Autoboot Camp", originally conspiracy crazed Cliffjumper was to be the Autobot framed for being a Decepticon spy, but Hasbro did not approve of that and that role went to Wasp.
The second Allspark Almanac goes in depth about what Season 4 would have been like - and it sounds like it would have been the best thing in the world, too. No, we aren't going to give the details, get the book yourself. note If you really want to know, we've got three words for you: Shattered Glass Animated.