No White Walkers
The White Walkers provided ice-cold malice to the original series. They won’t feature at all in House of the Dragon because it has a very different storyline. While the Wall exists in this timeline, it’s unlikely to feature very often if we see it at all (via CNET).
This is a shame because these elements of Westeros’s far-north enhanced the world. Fans will deal with a more contained series than the vast scale of Game of Thrones. This may suit the storytelling but it removes some of the grandeur of the original show. It has a lot to live up to.