The problem is sometimes things appear to get missed. Let's take the druid forms as an example. The old druid cat and bear forms were around long past their prime before finally being updated. The aquatic and regular flight forms for both night elves and tauren have the long, elvish ears instead of each race getting something more unique like the art above. The land travel form itself is nothing more than the cheetah monster model without any tweaks for either current druid race.
Another example is our old enemies, the murlocs. In Wrath we got cousins to the murlocs know as the gorlocs which have a newer and shinier model than their non-arctic brethren. We've even seen a few glimpses of deep-sea murlocs coming in the expansion which are also up to the modern modeling standards which put the old murlocs to shame.
Hopefully, all of the old art will be brought up to what we're come to expect for new content. It would be a shame for something to get left behind. Which things do you hope they'll update the art for?