I know the writer was in a bit of a bind, not being sure whether the series would continue and having to come up with something that would make a neat ending if it didn't, but could be manipulated to go on with the story if it did. However, there is something lacking in the whole finale - including the previous episode. It just seems to take an awfully long time to get where it's going and there are a lot of inconsistencies. The reason the book was able to be taken was because the sisters doubted themselves - yet once they realised what was going on that doubt disappeared. They seemed pretty confident to me about what they were doing, even if they didn't know exactly how they'd do it. And in previous episodes, when some evil thingy has managed to get hold of the book it still couldn't leave the house because it was connected to that too.
And Zankou just takes far too long to actually DO anything. Sure there are fights and all that good stuff, which is great, but having absorbed the Nexus, only to have it cast out of him, he seems to think that spending a lot of time trying to get the witches powers is the only way to go. He doesn't need those powers to take the Nexus, he could have easily have used the power-stripping spell from the book, which is miraculously still in there despite having supposedly been destroyed a couple of seasons back.
And when he is tricked by their astral projections - despite having Phoebe's power, which is supposed to help him know what the witches are planning - he doesn't just zap to the attic - no he gives them time to get there, look for a spell (which they don't seem to find before the SWAT teams arrive that they spend ages staring down at despite more pressing matters) and only then turns up to blast a hole in the ceiling and roof, giving the Charmed Ones somehow time to find a place to hide and sleep so they can astrally project again, trick him again and blow up the basement.
And that 'huge' explosion doesn't even damage the flower beds!
No, it was good and it was exciting in places, and the touching scene with dad when they think they might not make it back is a nice tough. But overall it ended with a fairly loud whimper when it needed a great big bang.