Well, we're only two episodes into the fourth series so far, but Doctor Who is off to a good start with two very strong episodes. I'm not sure about next week though; while I liked the Cthulhoid Ood back in series two, their return looks a bit like a cheap "aliens attack" story. We shall see.
The new theme tune is great, too. It adds a bounciness to the existing New Who theme, and there's even a sly homage to the Peter Davison-era theme in there.
Anyway, the writers have apparently been seeding this new series with clues as to the big threat coming up in the thus far untitled two-part finale (I hope it's not Davros; I'm bored of Daleks anyway, and they'd long-past exhausted the story potential in Davros by 1985). Here's what I've spotted so far:
The Shadow Proclamation. First mentioned way back in the first Ecclestone episode, and again in Tennant's first full appearance as the Doctor, it's been mentioned once in each of the two episodes so far.
Venus. In the first episode, Donna (Catherine Tate) and her grandfather discussed the planet Venus, and there was some attempt to equate the lone "female" planet with Donna's sense of individualism. In episode two, while dressing up in a Roman toga, Donna jokingly claimed a resemblance to the god Venus.
Missing Planets. The space nanny (yes, really) in the first episode chose Earth as a crèche for her charges because her own "nursery planet" had disappeared. This mysterious disappearance confused the Doctor somewhat. In episode two, the Pyrovillians (really nicely designed magma/rock beasts with a vaguely Roman Legionnaire appearance) were intent on colonising Earth instead of returning home, because their home had also vanished. Again, the Doctor picked up on this baffling occurrence. Planets just don't go missing.
My current pet theory, based on the brief appearance of Rose in the first episode, is that she's using the resources of alternate-Torchwood to try and get back to the Doctor. Back in "Doomsday", the Doctor burned out a sun in order to generate the energy needed to speak to Rose across the dimensional barriers; what if Rose is blinking out whole planets in order to get across? What if Rose is the villain?
What if Rose is Galactus?
Of course, it doesn't really work; why would she be munching up planets on our side of the barrier in order to get through from her side? Wouldn't she already be able to cross over if she could steal planets?
It probably is Davros. Sigh.
EDIT: The Medusa Cascade. I'd overlooked this one originally; the words stood out in the historical Pompeii setting, but I'd put them down to some oblique reference to the petrification effect plaguing Pompeii's soothsayers. It turns out that the Medusa Cascade was mentioned by the Master in last series' finale, "Last of the Time Lords"...