"But aside from the new third tier career for Wizards, we had some tough decisions to make."

Only the 3th tier career for Wizards? If so, I'm a bit disapointed... What about you?

They can only pack so much into each expansion. I believe they are just trying to flesh out the Empire for the most part, seeing as the core game takes place there. I would appreciate seeing some of the demi-humans getting some love. Though it seems every time they do that, they make a basic career that out performs any others. They have only released what 5 of the careers coming from this expansion are, so there is possible hope for a hedge wizard or something. They at least have a chapter about them in the book. I am personally happy the way things are, High Elf magic is mostly just annoying and imbalancing. Let the Empire have its' day.

They can only pack so much into each expansion, but an explicit magic expansion sounds like the obvious place to include magic careers. Considering they're also including several very non-magical careers, I'd expect them to have run out of any magic careers they could possibly include.

Makes perfect sense from a marketing standpoint. They've included info on Tzeentch as well as the other non-magical careers to make the supplement appeal to a wider player base than just those that play (or might want to play) a wizard. Meanwhile, by withholding the 4th tier wizard career and spells, they've got another product (or part of one) that is a guaranteed seller for anyone wanting to continue past 3rd tier. While this later supplement won't be out quickly enough for some people, it also avoids things getting too unbalanced with all things magical out to 4th tier while other vertical careers (priests, slayers) are stuck at tier 2.