Not a whole lot of information in that article. Musk has spoken a little more in depth on the Model X design challenges in other interviews/Q&A sessions. Since I have not seen any of these for a while please forgive and correct me if my memory serves me wrong. Things I remember him speaking about are:
1) tweaks that most owners would not notice, but would make a significant difference,
2) making the aesthetics subtlety even better than what we have seen,
3) increasing ride height and clearance above the Modal S while still achieving reasonable handling and efficiency.
1 and 2 were so related that maybe they should be the same point. 3 might be more projection based on what I hope the Model X achieves above all other CUV/SUV's before it.
I am in area where road clearance is truly needed on dirt roads at certain times of the year which is why I own a CUV. However I hate driving that vehicle because it corners terribly around the twisty roads due to high cog. The Model X has the potential to blow all competitors away in this regard by creating a much lower cog and hence better cornering vehicle than other CUV/SUV's while still providing adequate ground clearance. The challenge of increasing clearance while not taking too much of a hit in handling or efficiency is huge.
I am glad that the Tesla engineers have gained some real world experience via the Model S with customization and optimization of air suspension systems which I believe will become key to solving the above issue for the Model X. If the Model X does not run high enough by default, the air suspension system will need to do better than just temporarily raising up for driveways and parking. I would want the Model X to be able to go at least 35mph in a raised state while still being able to corner well.
But if you want to know what the real delay is about; It's the HC (hover craft) mode their developing to replace air suspension. It even senses large objects underneath and automatically raises the vehicle temporarily to clear them. I -- know -- I remember him talking about that somewhere. It's funny how posting about ones memory of what Musk "said" can serve that dream/wish fulling function so well.