George Lucas gets a lot of guff for the prequel films in the Star Wars saga. A fair potion of the criticism is spot-on in terms of his directorial choices, especially in The Phantom Menace and Attack of the Clones, but one thing that can't be criticized is Lucas's enormous vision. He imagined a rich galaxy that spans from worlds from the Outer Rim to the Inner Core. Up until 2008, most of the galaxy was gated behind novels and video games that weren't easily accessible to the casual fan, let alone children. And then, thanks to the talents of the fantastic Dave Filloni, The Clone Wars animated film hit theaters in a limited run with a companion television show by the same name.
To many, the film didn't resonate through their original trilogy nostalgia glasses, and the show followed suit. Well, for the first two-and-a-half seasons, at least. Now, Star Wars: The Clone Wars is regarded by fans as one of the best installments in the ever-expanding Star Wars post-buyout canon. It's plain to see, Star Wars: The Clone Wars is the best Star Wars anything since The Empire Strikes Back.
While it was alluded to in the prequel films, it's difficult to fully comprehend the number of strings Palpatine controls over the course of the entire saga. Not only is he deeply involved in the active manipulation of both sides in the Republic/Separatist conflict, but we get to see numerous subtle nudges he makes to ensure Anakin's eventual fall from grace.
Further, once you realize that he's effectively weighing the options of "which massive army is the most effective to take the galaxy over" with the Clone Wars, the entire situation becomes dizzying. The lengths some people traverse to gain power.
The lack of any real sort of relationship between Obi-Wan Kenobi and Anakin Skywalker was one of the weakest elements of the Star Wars prequels. In A New Hope, Obi-Wan tells Luke about his father. He says that he was "a cunning warrior and a good friend," but we never get to see the depth of this relationship in Attack of the Clones or Revenge of the Sith.
With the exception of Anakin and Obi-Wan's goodbye scene in Sith, every time the two are on screen together they argue or throw shade at each other. They end the scene frustrated and go off for the rest of the films on their own solo adventures, before reuniting for a fight at the end.
The Clone Wars fixes this. At the core of (nearly) every episode is the Kenobi/Skywalker friendship. The chemistry between Matt Lanter's Anakin and James Arnold Taylor's Obi-Wan is, frankly, light years ahead of what Hayden Christensen and Ewan McGregor shared with each other. It isn't hyperbole to say that watching the animated series between Episodes II and III makes Lucas's story much, much better.
In arguably the biggest addition to the overarching Star Wars story, we learn in The Clone Wars that ill-fated Jedi Knight Anakin Skywalker had a Padawan - a young Togruta girl named Ahsoka Tano. At the time of the series, audiences assumed the Star Wars films were effectively done - Episode VII wasn't a remote possibility for fans. Therefore, this new character, seemingly designed to bring in younger fans exclusively for the show, would eventually have to die. Or disappear. Or something.
A number of people actively rooted for this to happen, sooner rather than later. This all changed around Season 3 during the Mortis arc. Ahsoka Tano grew into not only one of the most badass characters in the series, but in the overall canon. She uses two lightsabers simultaneously in combat, led rebels against the Galactic Empire, and went toe-to-toe with Darth Vader, her former master. She changed preconceived notions of what a great character is in the Star Wars universe.
One of the best (and worst) things about watching The Clone Wars is the attachment and love viewers develop for the various clone troopers that appear throughout the series. In Attack of the Clones and Revenge of the Sith we meet, by name, exactly one: Commander Cody. This was, in part, due to George Lucas's decision to exclusively use CGI characters for clone troopers in the films. Lucas more than rectifies this in the series.
The level of depth achieved with the featured clone troopers in The Clone Wars is a gift in two parts - direction by Lucas and Dave Filoni and the insanely impressive voice work of Dee Bradley Baker, who voices every clone that appears in the series and manages to give them their own personality. Considering how things go for the clones in Episode III, the animated series makes Palpatine's Order 66 that much more tragic.