Lovely, a thread of the old days, without the attempt to troll or insult casuals, Thanks OP!
To answer your question,
I did raid at Vanilla up to Naxx, in TBC all content clear pre 3.0 and in WOTLK I cleared up to LK HC 10. Cata I did raid the first tier, since then never again.
Why did I stop?
For starters, all those different raid difficulties have increased the needed time to raid, time I don´t have.
But also the menthality has changed a lot. In Vanilla, I had a single addon, CTraid in TBC, I had a single addon for my buffs and a better layout for my heals in WOTLK I had a single addon for my tanking to taunt.
I cleared all content, did raid lead and nobody ever complained or asked me to install other addons. Not even Voice chat was used.
With Cata all that changed, people were super picky about who to take, you had to be in voice chat, had to have several addons and a certain build or DPS / HPS number.
I wanted to raid with like minded people, together where everyone can come and this no longer was possible. It became a second job to meet all the demands of people and I lost my fun in Cata, even though we were amongst the first to clear the content.
Gaming is fun for me, not work and that´s why I retired from raiding, just like I retired from group pvp after making Rank 14 twice in Vanilla.
Would I still like to raid, well yes. I want to see the story and improve my characters gear, to become stronger. (I love solo content for transmogs). But I also want the comradery and ability to just relax from RL. Running the same raid in various difficulties, with robots that are rude to people, just doesn´t fit there, so I remain inactive.
So ya, that´s my reason.