Finally saw this. It's good overall. The animation's good, sometimes reminding me of TMS's work. Excellent action choreography. The voice work is outstanding, the greatest strength here. Bruce Greenwood was an excellent Batman, though it struck me as odd that Bruce Wayne seemed to have a Western accent at times. (Doubly odd considering that Greenwood's from Canada.)
As for Jensen Ackles, I was skeptical, since I didn't care for him on Smallville and what I've seen of Supernatural. But he was startlingly excellent here, the standout of the whole cast. It was a note-perfect performance.
John DiMaggio as the Joker wasn't what I expected; I was imagining something broader and more operatic, and instead they went for a Ledgeresque, underplayed Joker. I'm not really sure how well that works for me, but he did do an excellent job at it.
Jim Piddock was good as Alfred, and the Martella brothers who played the young versions of Jason were good too, particularly the younger one, Alexander Martella (not only a strong performer, but one who sounded uncannily convincing as a younger version of Jensen Ackles). And it's always lovely to hear Kelly Hu's voice. But Jason Isaacs was a mediocre Ra's al Ghul and Wade Williams was unimpressive as Black Mask.
The main thing about the movie that didn't work for me was the writing. The story was okay overall, but Judd Winick does not seem to have a gift for dialogue, particularly not for clever or humorous dialogue. I probably would've liked DiMaggio's Joker more if he'd actually said anything genuinely funny. And Neil Patrick Harris, who should've been a standout as Nightwing, was largely wasted saying dull, obvious lines like "He's got lasers!" and "He's good." (His one really good line was the one about getting in the car, though that was more in the delivery than the words.) And there was too much expository dialogue, like Batman taking the time to explain to a two-bit crook who Amazo was, or Ra's al Ghul telling Batman what he himself had done as Bruce Wayne. The strongest dialogue was in the climax, the pivotal scene with Batman, Jason, and the Joker.
Some aspects of the story didn't work so well. Black Mask was an uninteresting character, just some loser who screamed a lot and said stupid things and punched people just to show how mean he was. A crude, unsubtle, annoying characterization. It seemed they were going for humor with him, but again, apparently humor is not something Judd Winick does well. And the whole Amazo thing was pointless. Of all the possible adversaries for Batman to face, why Amazo? Why not use a character from Batman's own rogues' gallery? An orange-skinned, pointy-eared robot that absorbs superheroes' powers just doesn't fit into a gritty, urban Batman story like this.
The music by Christopher Drake was pretty good, but I don't agree with earlier comments that it was emulating the style of the Nolan films' scores. It was rather more melodic. In the scene where the Batplane was chasing Red Hood's convertible through the tunnel, Drake was basically emulating Jerry Goldsmith. And there were a couple of points in the Batman-Jason fight leading up to the climax where the score sounded very much like the opening bars of Shirley Walker's Mask of the Phantasm main title cue.
So all in all, one of the better DC Universe movies, but alas, not because of the writing. It's not as inanely written as Public Enemies, and there's a good story at its core, but it could've had much better dialogue, and more satisfying secondary antagonists, than it did. Overall, it's the voice work that does the most to make Under the Red Hood enjoyable, followed by the animation.