Kara is my goddamn JAM. Hell, I always end up running through the entire instance when I Jump in each week to farm the mount just because I love the atmosphere so much, despite having the pet and all the transmog gear I could want from there.
To me, TBC just had the perfect balance of raiding difficulty, maybe up until Sunwell.
Granted, I was only 15 at the time of raiding, and I felt like I was pretty good at WoW for my age, so I may be a little biased. But getting attuned for Kara felt like a great achievement, farming heroics to be geared enough to get into it, then going from learning every fight and taking almost 3 nights to fully clear it at first until finally being able to tear through it in one night as a guild truly felt like you were getting better at the game. Then contiuning onto Gruul and then SSC just truly felt like you were completing a challenge to meet the next.
You would have to do Kara runs once a week to gear up guildees, but earn badges for yourself even if you were overgeared. It just seemed like a well designed system that allowed you to actually have to learn fights and be dedicated, but would allow you to "fast track" casual members of your guild and whatnot once you progressed far enough into the content.
I quit raiding after that, so I never got deep into TK and never hit BT/Sunwell. But it only seemed like the trend continued until 2.4 hit, where Sunwell seemed a little to nuts at first.
TLDR;
TBC had its raiding progression perfected.