I started playing WoW couple of months after TBC was released, right around Black Temple patch and can't speak of how intense previous content was but for my money it doesn't get better than Black Temple. My favorite instance by far. When I hit 70 (at least I think that was max lvl in TBC
) I thought Kara was really hard and impossible to beat and had no idea there were bigger instances in the game. But as I started playing more I started hearing people talking about these SUPER HARD 25 man raids that have the most epic loot and to be honest that's what kept my interest in the game. I started doing research about my class (Mage at the time), all the instances, what the best gear was available for me to get... I remember farming motes of fire for like a month (5-6 hour per day) to craft that Fire Set that Mages used. I just kept my interest in the game. Eventually I got a try out with the top guild on the server (only guild that was clearing SSC, TK, and Hyjal, 2nd closest guild was working on like 3rd boss in SSC
). When I eventually earned my spot on their 25 man roster that was by far the most fun I had playing a game. Progressing through Black Temple was really hard at the time but never the less a lot of fun. I remember when we got the first Illidan kill people on the server started freaking out and I was getting spammed with tells from people that I knew from my old guild of how on Earth did we kill him, what was the fight like, what loot did we get...
Now you can just que up randomly and go kill the hardest boss in the game in like an hour
I stopped raiding shortly after WoTLK came out and focused mainly on PvP. I did a lot of Arena through the entire x-pac and up through Cata which was a lot of fun if not more than raiding. But even that started to get old as there were no more challenges left in the game and most people have left by now and so did I.
Sorry for the long write up, but yea Blizzard started catering WoW a lot more towards casuals after TBC which drove a lot more players away. There was just nothing left to keep your interest in the game anymore. You can form a pug group on the server, go clear all the instances in the matter of hours and be left with nothing else to do for the rest of the week. Before you had to plan out your week as to when to farm for your flask/potions mats, farm outside main raid for other gear, prepare for main raid and possibly new boss... It was a lot of stuff to keep track of but in the end all the hard work paid off and that kept the game interesting for me for a LONG time, but those days are long gone now