Lately the question of whether writers can write animated cartoons -- always a hot and heated topic online -- has been coming up for discussion again. See Mike Barrier and Mark Mayerson for two examples.
One of the better posts taking the anti-script position is this Eddie Fitzgerald post. He points out what is true not only of animated cartoon scripts but all scripts: they tend to emphasize dialogue over everything else because dialogue is easier to read. As an example, he presents a couple of pages from an unproduced Animaniacs script he wrote.
Stage directions just aren't fun to read; they're flat descriptions of stuff that might happen in the show, whereas the dialogue is, literally, what's going to happen in the show. There's no doubt that there's an advantage to presenting the story in pictures instead of just words: you can actually give a real idea of what is going to happen onscreen, and make it enjoyable.
Incidentally, this is probably the reason why most of the writers who did get a lot of material accepted for Animaniacs were improv comics from the Groundlings and such. (On one of the DVDs, Tom Minton, one of the few writers on that show with an actual animation background, takes a sardonic dig at the invasion of the improv people: referring to himself and other animation people, he says: "We weren't hip, you see. We weren't the hip people.") Being a performer gives you an advantage in pitching a story because you can act out the visual stuff and make it sound funny. Artists are supposed to be able to do that by sketching out the story and presenting it that way, but that's not an option that's available to them in much TV animation, where you just submit a script to read.
Fitzgerald's comments section also includes the inevitable contributions from John Kricfalusi, who as always seems to assume that all animation writers are hacks and frauds because he worked on some bad shows at Filmation and Hanna-Barbera. He also seems convinced that because Fitzgerald is a cartoonist, his Animaniacs script is better than any of the regular writers' scripts, which, frankly, I don't see.