For the first time we really get things that would be almost impossible to visualize on television, even with today's effects. We also get multiple layers of plotting and time looping and other timey-wimey stuff, which is where I have to admit I always go off the rails with time travels shows. Normally, I don't really care. But this book makes me want to trace the layers, if only to see how it's done and applaud.
And we get characterization, thank goodness. Some of the characters are more lightly drawn than others, and some come from earlier books and assume we already know the drill with that particular character, but all are at least cursorily fleshed out and given motivation. Ace is angry and loving and scared and brave as hell. And, of course, we get multiple Doctors.
I am normally leery of multi-Doctor setups. When they're done well, they're incredible (it's well-known to my friends that Time Crash is my favorite episode ever, at all of seven minutes long). The problem is that when they're done badly, they're appalling. The Timewyrm tetrad of books has already seen the bad. This time, praise
Before this gets too glowy, I do have a very slight problem with the Christian bent of the book, but it's a nitpick about one character in particular and it doesn't distract from the whole too much.