I think most of the anger and disappointment with the X1 over the resolution issues aren't just about the pixel disparity right now, but what it could mean for the future. Will this mean smaller levels on X1? Less physics interactions? Worse graphical effects in general? Worse AI? A lot of it directly affects gameplay, or at least immersion which is a huge part of gaming for a lot of people. What happens when the PS4 version of a game is so big or graphically intense that it has to run at 720p/30fps? How will it run on X1?
I think it's the startling performance disparity in general that we're seeing already that's the main issue, not just the resolution. With the premium pricing taken into account, it's no surprise that it's kind of a big deal.