I see it as this:
A hardcore game is a game where a casual player has an extremely hardtime succeeding, but the hardcore gamer has something they can play without running out of content. I feel Old XI (pre-abysea) accomplished this, I played for over 1000 RL days of playtime since launch and I never ran out of things to do, there was always something i could work for, something I could get that was better I could improve my character with. But the design of the game is what most players (casual base) see as not good, waste of time, too much work not enough reward, etc.
A casual game is where a hardcore player will just fly through the content and get stuck in the waiting game, but the casual gamer will be able to maintain a steady stream of progress throughout their playtime. I feel WoW and NA mainstream accomplishes this casual players can succeed and can easily progress through the content. They are rewarded for ever second of playtime, it is focused on little work but high reward. But the issue is the content... it is all short lived and quickly replaced. For example I started in Cataclysm on a new character.... I almost got to the new level cap in 5 days (seriously that is almost consol leveling curve), I have some friends who got almost all the new dungeon gear in 1-2 months they are now waiting on new content patches to be able to continue. {So basically WoW does not have enough content for all its players, 1 of XIV's issue atm.}
Someone who was just a casual player, played only for fun when they had free time, didn't want to really worry about grouping and building a strong bond with other players... honestly didn't do well in XI at all. They might of got to endgame probably took a few years but that was about it. They couldn't/didn't want to dedicate time to join a endgame shell, They didn't want to have to schedule the game into thier lived, They didn't want to have to wait 2-3 years for gear because they could only play 8-12hrs a week.
In a game like WoW that same person would get all the way to endgame, can do the easier raids to get decent gear, and so on. It is why WoW (the first casual mmo) was a massive success. There are simply more gamers who are casual and WoW turned the Hardcore genre into just another genre for all gamers. This is the same reason why so many hardcore oldschool gamers really dislike WoW.