Firstly, increased detail levels in the scene is likely what is causing the issue, along with farther view distances. Far views result in more aliasing-related issues with trees and such in the far distance, as well as aliasing with the high detail stuff nearby. It results in a sort of noise being applied to the scene if not dealt with entirely, and is likely the cause of many of the complains about modern graphics.
As for FPS, there is actually a more important quality to it than just the FPS number. And that quality is stuttering. If I have 100 frames that render in 0.005 seconds, and 1 frame that renders in 0.5 seconds, the result is a massive stuttering that makes a game unplayable, despite only lowering the displayed framerate to 30FPS. This is actually the real culprit behind any of the 'unplayable at <number greater than 25> fps.' When computation isn't evenly spread out between your frames, you end up with say, 2 frames having a speed of 10FPS and 36 frames having a speed of 60FPS. That will be extremely noticeable. Keep in mind, FPS only tells part of the picture, since the actual frame times are highly non-uniform if not addressed properly. Those examples given above provide you with 30 and 38 FPS, respectively. If those were entirely uniform, there would be no issues nor a noticeable frame rate slowdown; but with the numbers making up those frame rates, it is not only highly noticeable, but would degrade your gaming experience greatly.
FPS means little in regards to actual perceived frame rate, which is much more strongly influenced by stuttering.