Toxx, the rules that have been changed that affected me the most are based around the ease of the game. I remember the days of the ogres in Loch Modan. I enjoyed the elites that were dangerous. I remember the days of wondering what I was getting myself into when I went into a new zone. There were no easily placed level limits on the map, things were not ao streamlined. Wow was not a hard game per se at launch, it did however require a breadth of knowledge once you got into it.
I feel like Wow at vanilla was a sandbox with many themepark elements being built at the time. Now it is just so settled, there is no adventure left that has not been weighed and measured to the nth degree. Pandaland was very rote, polished yes, even fun the first time through, but like some zebra stripe gum I got a month back out of nostalgia it lost it's flavor very very fast.
The rule that change for me was vanilla wow had more character, it had sharp edges and prickly bits that could snag but it was part of the charm, the wow of today is polished mashed potatoes, still good but just mushy soft through and through. (yeah yeah heroic raids!, I was never much of a raider, even though I did it for years, it is not my preferred gameplay.)