The main issue with Timeless Isle is that it's a zone based around exploration... which is also very small. These two facts represent a contradiction in design philosophy which is highly detrimental. Choosing to experiment with a new form of content while simultaneously outright stating that the zone designers are busy with next expansion stuff didn't work out so well in retrospect. For an exploration-based sandbox content hub to work, there simply has to be a ton of different content spread across a large zone. The Isle of Thunder would have worked fine for open-world exploration, for instance, as there is simple far more locales there.
As it stands, Timeless Isle is way too small for what it attempts to be, and thus ends up feeling like the devs awkwardly attempting to feed us a halfcooked spoonful of the delicious meal they'll be serving next expansion when they have actually developed larger areas around the same sandbox design principles.