That is so not how it works. Look at all the Blizzcon screenshots there.
I wish someone would do this for every expansion. Differences between Blizzcon and Launch. There's always something.
I'm gonna bet that Transmog system comes out with 7.1
Okay, artworks and videos of datamined unfinished stuff never shown at all.
Some stuff will be delayed as usual, and more canceled stuff will be found in the files, as usual. Now the only big catastrophe would be if they decided to scrap scaling, but the whole xpac is dependent on it (as well as challenge mode) so the risk is small.
I don't believe they have shown stuff that they will cut (although I expect the traditional island on the map to mysteriously disappear) because they have shown zones in video, dungeons in a reasonable number (so the usual package), and end game system that are trivial to put in place.
Challenge mode is just numbers, adventure mode is just more quests. Nothing there is particularly complex or hard to achieve.
The obvious delay candidate is indeed the transmog system, which is a complex thing, even if they are working on it since WoD.
My opinion is still that if things are thrown out it's because it won't go well with the balancing, so class and artifact stuff (also less pvp perks than announced I'm sure of it). I'm sure these will come back to bite them in the ass.