Dalek

Not sure. It's never made clear why the Dalek absorbs Rose's DNA and how that causes it to mutate. It's generally cheating to make the main baddie decide not to be a baddie at the last minute, too. Still, it is at least something that happens gradually throughout the episode. The Doctor seems to stumble on the Dalek-killing gun a bit too readily - it would've come in handy in Parting of the Ways, no? And if he's abandoning pacifism why didn't he stow it in the Tardis for safe-keeping? Oh, I dunno.