Maintaining proper sightlines (especially views of outside the park but also between lands) was considered important from the beginning.
That's why a berm was built around Disneyland. That's why Walt conditioned his selection of Anaheim upon a zoning restriction that prevents the development buildings tall enough to be easily seen from inside the park. That's why, before opening New Orleans Square, Walt had a tree planted in a specific place to block views of the Matterhorn from that area. And so on.
This does not mean Disneyland met this ideal 100% of the time (e.g., before the berm's trees grew in, one could see power lines that were outside the park, and of course the tops of some structures in various areas of the park were visible from other areas). This also does not mean they didn't allow exceptions based on balancing other considerations (e.g., seeing outside the park from Matterhorn Bobsleds, and seeing the tops of facades from the Skyway). But it does mean that they considered sightlines integral.
Furthermore, Disney placed even greater emphasis on sightlines when developing the parks that are most highly-regarded for their design quality - Disneyland Paris, Disney's Animal Kingdom, and DisneySea. They took sightline principles that were developed from the very beginning, and - with larger budgets and more highly-developed design skills - they improved upon their execution.
Sightlines are an important part of what makes the Disney theme park experience work. I hope Disney does not falter further on this central principle.