The killer disappointment for me (with MoX) was the disturbing lack of xenos...pls allow me to explain!
Loved the tau stuff, as i know next to zip about them - great! This made the book for me, right there.
Good tyranid stuff, not all of it, but a good chunk - nice! THis was my yummy gravy.
Good chaos stuff, always need chaos - nice! Always handy to have greater demon stats.
Xenos....some eldar, waaayyy less ork, no 'crons (to be expected given the setting, but....game shouldn't limit its material to just one setting - but that's not the argument here, so let's keep reading and ignore the lack of necron nastiness for the purposes of this thread)
But the real big kick in the pants is that the xenos section includes, perhaps, a dozen new aliens for us to play with. A dozen? Really? That's it? Myself, and I can only speak for meself, I xpected a book called Mark of the Xenos to include, ahem, a few more aliens...to be fair, Chaos could have been left out, as they already got a feature in the main book...orks should probly have been in the main book too. I really think the major bone of contention is the underwhelming number of aliens in the monster book...FFG could have dispensed with the librarian's notes and fancy bricabrac they load their products with, and kept things trim, stream-lined, and crammed with more monstery goodness. I must admit that, given the other wh40k lines, i thought we'd get a whole helluva lotta cool aliens from all the different lines, statted for DW and ready to eat space marines....
Now, I'm a veteran of the grimdark, so i can stat critters with little real problem...but, for someone who has not waded thru the Emperor's realm for 25 years, the task of monster-building could be a tad overwhelming...it is they who needed a comprehensive xenos guide...it is they who were left in the cold with no critters to fight...it is they who look at the rulebook and monster manual and say: "Is this all there is? Is there nothing more?" Heck, a space marine can't even kill a grox, cause we ain't got stats for it! (grox = cow, by the way; giant lizard cow, i s'pose, but cow nonetheless)
So, is it the customers' fault for having expectations? Is it the customer's fault for having expectations not met? Me, I'd say no to the first question, qualified no to the second...