Say you load a very long article, and start reading it. In the middle, you want to do something else, so you press the Start button and get to a different app, leaving IE behind. Once that's done, you get back to IE.
In WP 8, the long article you were reading would be waiting for you exactly where you stopped reading, no matter how long you spend outside.
But in WP 8.1, if you spend even 15 minutes with the phone locked on the start screen without opening any other apps, as soon as you get back to IE, the page will reload. Now this itself is okay, but the problem is that when this happens, most of the time, the page will go back to top, and you will lose the point where you were last, which is a great annoyance for some of us who read large texts on the phone's browser.
And if reading mode were to be on when switched to the start screen, chances are that the page won't load at all when you get back, leaving you with a blank screen, forcing you to exit reading mode and refresh manually.
It's also absolutely horrible when dealing with large pages, though it's still better than IE 11 because it doesn't have the reloading issue, at the least.