In general - a good, solid episode. Not stand-out brilliant, but of course by definition not every episode can be. Alongside them, you also need episodes which just gently and competently move the series forward. If this represents the base-line of ordinary episodes for series 4, we're on firm ground.
Donna - still developing very nicely. She asks the right searching questions, and spots the flaws in what she sees. But she's also saved from becoming just some walking paragon of moral rectitude by her very human limitations - being thrown off guard by the first Ood she sees; wanting to go home when she realises what the 42nd-century humans area doing; knowing that she can't handle listening to any more of the Ood song. So long as that balance is maintained, and she doesn't just become the voice of 21st-century western values, she stands every chance of becoming my favourite New Who companion to date. Also, putting her in a furry hood on an ice-planet was very Romana I. Good work.
The Ood - fundamentally, I liked what this story got out of them. But, although I am no evolutionary biologist, I did find it pretty hard to suspend my disbelief about them evolving on an ice planet. We don't know what their bodies are like, since they always wear clothes, but we do know that they have large, hair-less heads, and lots of tentacles - which would have a high surface-area to volume ratio. And that, of course, is before we even get to their external brains, which were shown steaming in the icy air of the planet when the Ood opened their hands to reveal them to the Doctor. I know the brains were meant to be a plot point, explaining their peaceful nature, but still their overall appearance seems designed for body-heat loss, rather than body-heat conservation - more at home on a desert planet than an ice one, surely? But hey, that's just me being a pedant.
Speaking of pedantry - also, the guy who got swallowed up by the giant brain? I'm not sure why his story was included at all, given that it was revealed and dismissed in about two sentences. Or the bit about the Ood turning Tim McInnerny's character into one of them. Also, how did Mr. Halpen know that throwing the Friends of the Ood scientist down onto the giant brain would result in his death? Surely he'd be just as likely to bounce off and be fine? (I suppose the fanwank answer is that Mr. Halpen is so evil that he's done it before, for fun).
Wider arcs - two things stood out for me here. 1) We were given quite a lot of very specific information about the Second Great and Bountiful Human Empire - viz., it's the 42nd century (4126, to be precise); the human empire extends over at least three galaxies; and Ood Operations began exploiting and breeding Ood for slavery around two hundred years previously (i.e. in the 40th century). I'm just noting all this down because the development from The Long Game to Bad Wolf, from New Earth to Gridlock and indeed from Utopia to The Last of the Time Lords shows that when we are given this much information about the future development of the human race in New Who, it tends to be because that information will come back later on.
2) Telepathy was a strong theme in this episode - both the Ood's and the Doctor's when he helps Donna to hear the song - as it had been in Fires of Pompeii between the Sibylline sisterhood, too. Obviously, it's always involved in Doctor Who, but it just seems to be bubbling up particularly strongly at the moment. I'm guessing it's likely to precipitate a major plot development later in the series - like the Doctor telepathically sensing the presence of his daughter, or using it to communicate vital information when he can't reach people any other way, or something.