I also like tying the sourcebooks into the storylines, so long as the storylines are suitably thematic. Elemental evil is pretty thematic for D&D, so I think it will probably be a good sourcebook. Has some decent developer names on it too. I could see problems if the storylines get more tightly "Realms focused", but if they can stick to iconic D&D storylines like "dragons" or "elemental evil" or "demons", I can see it working well.
They expect to kick off the next storyline in March - about a seven month gap from the kick-off of the Tyranny of Dragons storyline. That only gives them about 5 months until GenCon - will they kick off a new storyline there next year as well? I keep wondering if two storylines a year will be too ambitious. (It's far more ambitious than my group would ever be able to do, but we are old and can only play about once a month - or sometimes ever other month - so I suspect we might be outliers). I'd really like to see a second world in there - Ravenloft or Eberron or Dark Sun or Planescape or something. I can see how you wouldn't want to split your organized play like that, but I'd still like to see it.
And Princes of the Apocalypse is $50 instead of $30 (for Hoard of the Dragon Queen) The whole Tyranny of Dragons adventure is $60 between two books, but the way the ad copy is written I would kind of expect another book in the series. That's a bit of a shift - I wonder if the book is significantly bigger, or if the cost on the two Tyranny of Dragons books was lowered to make it attractive to jump on as a starting point for the new edition.
(The ad copy is also kind of annoying "Provides background and setting information critical to having the greatest chance of success." Just that one sentence- the rest of the ad was fine. You really don't need to do the hard sell of "BUY THIS BOOK OR YOUR CHARACTER WILL DIE IN THIS ADVENTURE". It's the second release of books for the game and it's going to be full of lots of bits and bobs for players - sales are going to be pretty good whether you beat that horse or not.)