WOW is the best selling game of all time and is ironically the best example of a game that started as a dumbed down game (not always bad) and was further dumbed and streamlined to appeal to the masses
WOW at the time it came out in the later part of 2004 was an extremely casualized and streamnlined MMO . Compared to shit like EverQuest and Ultima Online, the most successful MMOs before it, it was extremely casual . If you died in EQ you had to run back to your corpse completely unarmed and unarmored to acquire all your gear back. Assuming you died going down fighting with all your equipment and items, imagine trying to survive the same area completely naked or using welfare replacement gear. If you died in Ultima Online, people were able to loot your body. Someone could kill you and literally steal all your equipment.
Mobs had so much aggro in EQ and weren't "leashed" so if you pull one you had to run out of the zone to reset aggro. In WOW, mob leash area is only about 50 ingame yards or so. Aggro was so important that there were entire classes designed solely around crowd control. This meant that you could literally do nothing alone including killing simple mobs. In WOW you could get to max level soloing on any class. It wasn't fast or easy but it was very doable on any of the 9 playable launch classes. There was no instancing in older MMOs. If you entered a dungeon or raid, you had to contend with the players currently residing inside that dungeon as well as the normal AI residents.
WOW was a massive success because it harnessed the community aspect of MMOs, leveraged a popular IP, and cut out much of the insane time sinks and frustration features of MMOs that the average player didn't want to deal with.
Now WOW is currently 6th expansions in and it has been steadily removing more and more rpg elements each expansion.
Compare an older stat screen to a newer one
OLD WOW STATS
So many stats that you needed to scroll down to view them all. Need to use two images to display them all.
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MODERN WOW STATS
Really simple
Current WOW has made leveling up to max leveling a complete joke, questing mobs are never in danger of killing you no matter how poorly you played or how mediocre your equipment is. Classes have been pruned to toddler levels with a lot of complexity and of situational abilities have been removed. Dungeons are so easy that you can mindlessly pull everything and AOE it all down. Buffs have been either entirely removed or baked into passives
Honestly, you could write a small novel on the number of elements removed from wow
https://www.reddit.com/r/wow/comments/3g8nez/comprehensive_list_of_all_rpg_elements_removed/